How is it whe you glue 3/8" clear to 3/8" clear? Often times, black and other (especially opaque) colors are "harder" and simply do not glue as well. This effect varies from mfr to mfr.
I'd reall advise folks to get clear mastered prior to doing colors. At least with clear, you can see what's going on and use this knowledge and experience when using colors.
When you were doing 1/4", what brand were you using?
Try not letting it soak so long. You should not have to reapply solvent like that, esp something as thin as 3/8". Should be a "one-shot" deal
Temp is fine, humidity is fine. No worries there.
FWIW, Chemcast is at the lower end of the totem pole IMO, try using the same techniques on Polycast, all clear at first to start
While I know how the Chemcast is made and in theory - should be fine, I don't know who their resin source is or if it's a consistent source, so material may vary from batch to batch and there may be variations from one thickness to another.
IMO (again) acrylic really is one of those things in which you gt what you pay for. The better materials just cost more, and quite worth it
HTH,
James