Another cause can be if the right hand cylinder knob is seated too far inward and is binding without allowing enough clearance. I had this same problem with both my Underwood standards when I put the knob back on too flush with the area that allows the shift motion (carriage skeleton guide.) There should be a tiny amount of space between the knob and the carriage skeleton guide so that there is no binding. Here is an illustration:

 

I am happy to announce that I think I have solved my shifting issue. So what I did was press the shift key so that the platen assembly would be stuck in the upper position. Then I pressed down one of the pivot arms.


If it was binding up at the pivot then pressing this arm down should have dropped the platen assembly.  It didn't.

I tried doing the same on the other side and the left hand side did drop. So it was binding up on the right. I watched where it wasn't moving and tracked it down to the linkage between the platen assembly and the carriage.



This is the linkage in question. 



And here it is in a blurry photo from underneath.

So I removed the platen assembly and sure enough this linkage showed where it had been rubbing against the platen assembly.  So I cleaned it up and used a file to clean up where it had been binding.

Now it shifts effortlessly. I still plan on spending more time cleaning up the pivot points  and getting it all shiny. But now at least I can type on it.

Or at least I will be able to once I buy some spools that fit better.