Primer

I use both primers with no issues.  Are you giving them time to cure and is the plastic clean before  you put the primer on?

The best option would be to find somebody who likes the stuff and give it to them...

The biggest issue with that is that most dislike it due to the poor adhesion.

I too have tried laquer thinner to no avail. I'm still using my bottle because it's black and I can still use it as a somewhat decent base coat for some of my parts, but once I'm done with the bottle I'm never going back. I second the recomendation on giving it away to someone who likes to use the stuff. For me it just doesn't stick and peels constantly even on parts that I presanded and let cure for days in preparation. Other primers work way better.

VJ POLYEURETHANE primer can be 'hardened' slightly  by using US (see-through) Future from the transparent bottle.

I have NOT tried this with the UK  (milky-white/brown) Pledge in the white bottle, because I don't like the smell!

This will help slightly, but you still won't be able to rework it much, because it will delaminate in chunks & you can't feather sand the edges.

This could be for 2 reasons:
The plastic needs to be 'keyed' even if with a 3M Scotchbrite pot scrubber, '00' grade wet & dry or even 0000+ grade wire wool.
It also needs to be grease free, so wash in dish-soap/washing up liquid or 99% IPA & dried.

Also, if sprayed on too thick, it tends to take forever to cure & can easily come off.

Upshot: I keep for groundwork, or the German RLM colours I keep for top-coat, where there is no re-work/sanding/handling needed.

Upshot2: use Badger Stynylres instead, good colour range & much more forgiving of handling/rework/sanding etc....

PS, none of the VJ's  like any form of IPA or alcohol....