I Just solved this exact problem.
I've been having the same issues with my Alumilite clear resin. I mixed in a plastic cup, a paper cup, pressure, no pressure nothing was preventing it and it was a brand new batch. First time ever so I assumed I was doing something wrong. A bunch of emails back and forth with customer service yielded nothing. So, I remembered reading about heating the blank. That didn't work. So then I figured well I'll nuke the resin. So I put A side and B side in microwave for 20 seconds, mixed for around 30 seconds and poured. Wouldn't you know it first clear cast out of 6. lol Give that a try. It may be moisture in the A side or the resin isn't mixing well because of the viscosity. Don't know if it's cold where you are but it's a 20 second attempt that just may work.