Hi Jesse -
Took a look at the pic after reading your description of what happened. Two things - air temperature and prevailing moisture in the air and in your media are big factors for success. The pressure pot doesn't fix things, it just compresses the air left in the resin to very small bubbles that are not visible to the eye. In order for the pressure pot to work - which usually takes about 4 hours under roughly 40 lbs of pressure - the resin has to be working properly so it hardens some amount of time before you release the pressure.
Alumilite is exothermic - meaning it requires heat generated by the chemical reaction to set. Alumilite slow requires 50% of A and 50% of B BY WEIGHT in order to set properly - NOT by volume. The resin requires every molecule of part A to be in contact with every molecule from part B, otherwise the resin will not cure properly - usually staying soft or taking a really long time to cure.
So that might have been first possible issue. If the resin is not mixed thoroughly, or if when you scrape the mixing bowl to get the last of the resin out you picked up some unmixed resin, that could be your hardening issue.
Secondly, in the winter, it is advisable to get the resin to a warmer place prior to mixing. If your resin was too cold, the pot time may be affected, as the mixed resin needs to get to about 180 -200 degrees for it to set up properly. If your room is cold where you store it, mixed it, or had the pressure pot, the outside air may have negatively affected the resin. The chemical reaction could not compensate for the low temps where you cast.
Second potential is humidity - three possible sources - 1) your air compressor 2) your flowers 3) excess humidity in the air. Alumilite foams when it experiences contact with moisture. If you don't have a water/moisture trap on your compressor and if your compressor has moisture or water in the air reservoir, you could be getting the foaming from there. High humidity also has a double effect - the air itself has a lot of moisture, and when compressed, the air compressor amplifies the moisture through compression - mandating that you have a moisture trap on the output of the compressor when feeding the pressure pot. If you haven't bled your compressor storage tank lately, I would also look at that, as the tank can hold a lot of water without affecting normal air operations.
From the looks of the blank, I would say all three factors were in play - there was definitely moisture affecting the resin (from the air compressor the flowers), and the cure time was long due to mixing issues or cold shop.
I would get a moisture trap for your compressor, low temp bake the flowers for a few hours in an oven before casting, and monitor resin casting temps and give it another shot!
Good luck!!!
Kevin