I've been also having a bit of problem with the clear not totally setting up, but if I do the same with the white, it has no issues. I weigh my product before use and usually get great results, but the basement is a bit cooler than normal lately and was thinking it was probably that. I'll just put those aside and check them in a couple weeks...
You didn't say what you did, so here goes. Assuming you are using Alumilite Clear.
I store my resin in the house and take it to my unheated shop when I'm going to cast.( I do heat the shop when I'm out there)
Mix between "A" & "B" side is supposed to be mixed 50/50 by weight. I mix mine by volume 50/50 and have NEVER had a problem.
Once mixed it goes in the pressure pot at 60psi(Alumilite recomends 40psi), for 45 minutes. It is always cured by then.
Alumilite cures by making its own heat. If it is really cold in the shop when I start I do pre-heat the mold a little.
I could be wrong, but it sounds like you are not mixing the two sides long enough.
Remember you have 6-7 minutes open time and that is a long time. If you mix for 2 to 2-1/2 minutes, you still have 5 minutes to get it under pressure.
I also pour small cabochons (jewelry pendants, maybe 1" by 1/2" thick) in my cold shop. I do not put these under pressure and just let them sit there all night. They are a little tacky still after a couple of hours. After all night, they are cured.