Protein samples still in liquid form at -20C

[summer]

Dear folks,

Please help me out. I store my purified protein samples at -20C and they are still at the liquid form (a few are frozen) at this temperature! I used Trizol kit to purify them and dissolved them in a solution of 9M urea, 4% CHAPS and 30mM Tris/HCl pH 8.5 buffer. They had very high concentrations to start with, but with time the concentrations are less than before. Can their liquid form degrade them? How can i store my protein samples better? At -70C? I have asked some people and they store theirs at -70C.

Thank you all for your kind help!