Bad bad Inking

Those rollers look like they are rubber— they shouldn’t swell.
You can take some strips of the polymer plate and use it as bearer tape. Use the thin part where all the polymer has washed away. This will be much more effective than a large amount of strips of a thinner tape material.

Don’t give up yet. I think you just need to get it dialed in! Do you have a roller setting gauge?

DGM

How is the type you used on the sides inking? Is the plate higher than the type?

If the type is inking OK, then the plate is probably the culprit. If not maybe you are using too much ink?

Have you tried printing with the same setup but just the type; replacing the plate with furniture?

The photo is a bit indistinct, but it looks like you are using new delrin trucks and the rollers seem to be the same diameter, so I wouldn’t think the rollers are much of a problem. I only see two rollers. If you left ink to dry on them and didn’t clean up thoroughly, you’ll have some problems with inconsistent inking.

You need to start measuring things. Get a micrometer and calipers, learn to use them.
What is the base height? What is the plate thickness? Together do they come to .918”?
What height are the tracks? Certainly worn below .918” but how much? (On my last C&P, about .050”, which is a lot.)
What are relative diameters of roller and truck? In most texts, referring to forms of metal type and plates, trucks whould be smaller than rollers, BUT in contemporary photopolymer use, they should be the same diameter.
Use of type as bearers is not a bad idea, but just against roller slur. Even solid wide roller bearers won’t do much to lift rollers that are set too low. Letterpresss rollers are soft intentionally, and by the time they have hardened enough for bearers to raise them, they are too hard to use.

Use the thick plastic tape to really build up the rails

http://order.nagraph.com/roller-setting-gauges.html

no matter how uneven and worn, it’ll at least put you in the ballpark without inking everything in site.

I have a couple of suggestions you can try. Your photo is hard to see, but to me it looks like your PP plate and base are inking up. The type you locked up next to the base doesn’t look like the rollers have inked anything but the face of the type. If this is true, then wash up your press, put your chase with base and plate back into the press. Turn the flywheel until the rollers are over the base and plate. Try to slip a piece of paper in between the plate/base and rollers. Is it a tight fit? Is it a loose fit? You are looking to find out how much of a gap you have between your rollers and plate/base combo. When you put your type high .918 metal type next to your base/plate are they the same height? I think your base/plate combo is too tall (more than .918) You should try to tape your rails or trucks and see if that helps.

You might consider building up the rails with something like 2 pt. scoring rule which might supply you with a bridge across worn areas. To me doing it with roller bearers just isn’t a good solution if you’ve got this much of a problem. What we did to build up our rails was to take some good new type and lock it up in a chase. Then you need a serious metal straight edge you can lay across them to see where the rails come up to so you can see how close they are to type high. It may take an afternoon of playing around, but it should get you close at the very least.

What if it is simpler?
What if she has a deep Base for polymer but uses only a low polymer? Have seen it before, the bases for the 145-152 plates are lower than the ones’s used for .72 - .94.
Perhaps if not already done, the maker should use different colors to identify the bases.

You have to be a little smarter than the machine. Not a lot, but at least a little. You must understand how it is supposed to work so you can adjust or repair it to help it to do what it is supposed to do. Your alternative is to take the work to the copy shop in town.
Typenut may be on the money. You must have the proper base for your plate. Both the deep relief plate and its proper base come to .918. So too, the standard plate and the standard base come to .918. A deep relief plate on a standard base will be over .918 and the whole plate will ink.
The Boxcar standard base has black markings. The deep relief has red markings. The deep relief base and plates are not really to give much more punch into the paper. Rather they were to give a deeper beard to the type or graphic to avoid the inking of the non printing area. This for folks who could not or would not adjust their press properly. Almost all old presses have worn rails. You have to adjust them with tape on the rails or on the trucks to get the rollers to the correct height. Another method is to put .918 aluminum bearers in the chase. They support the rollers. They also take ink and the tympan must be cut away where they would print. Using the bearers is catering to the symptom rather than treating the cause.
It is difficult to put metal back on the rails and machine it to .918. Tape is usually the best quickest bet.
Some have lowered the bed of the press to the back (if a floor model) and built up the rails with Bondo and then filed them down flat to get them to or just under .918. Then the rail is brought to the correct .918 with tape.
Isn’t this fun? Or would you rather take it to the copy shop?

It is good to be a little crazy to work at this archaic craft. It is far easier to print with metal type than poly plates. The poly plate requires a very well adjusted press.