This is of no help, but PMMA (polymethyl methacrylate) is just the chemical name of this material. It seems in English acrylic is more commonly used, but it may refer to other products. Plexiglas or Perspex are trade marks.
Sorry if my post sounds harsh, no offense intended. My English is not polished enough to smooth things.
As Mobius noticed (BTW, with a smiley), except with a lot of luck, you probably won't be able to solve this without more "scientific" testing.
The problem seems tricky:
-you obviously mastered your process until now (and thanks for sharing it, this use of clear is very interesting)
-it seems nobody else on the forum uses it, so no practical experience to share
-at first sight, nothing has changed in the process and materials
It may also be something just under your nose and you do not see.
In such cases, I ask someone else advice. Not as you did on this forum with people who may have some knowledge. I just show the problem to somebody who is not aware of the job (an employee, my wife, one of my kids, a friend). I explain slowly, methodically and precisely the context, and the issue itself. Very often, the miracle occurs: while explaining in details, I realize which detail I didn't care of or I didn't see, or the newbies asks the right question or makes the right remark, putting me on the tracks.
If it doesn't work, you probably have no choice than using a more "scientific" testing. I'll add to Mobius suggestion another one: pick a piece of PMMA (sorry
) scrap which used to work and a piece which doesn't work and process them simultaneously.
Reading carefully this awful instructions in small characters on the rattle can (I hate them too) is not an option. You may discover something.
I also emphasize on hygrometry: if your clear is a PU monocomponent one, it polymerizes with atmospheric humidity, so much slower if air is dry. Dry air can also produce electrostatic issues which may cause the PMMA to repulse coatings.
Still no answer, just tracks.