I am new to CLARITY work. I made my very first batch of hydrogel monomer solution two months ago (4% acrylamide, 0.25% Azo initiator, 1xPBS pH 7.4, and 4% PFA in ultrapure water) following the protocol precisely. The polymerization was great at my first attempt. Then after several weeks I did another embedding with the same batch of monomer solution. Surprisingly, after 3-hour 37 degree incubation, the solution is still almost in liquid form and no apparent polymerization. The hydrogel solution was stored in -20 degree and thawed on ice. I made a new batch, but still no polymerization. Any thoughts on how to solve this problem or trouble shoot further?