Mobs dont spawn in the dark place and dont spawn in night or day in my mobfarm

Since the update 1.2.5 nothing spawn in my mob farm . Nothing spawn night and nothing spawn day . Earlier in 1.2.3 always spawn mobs there but than come the update 1.2.5 and nothing spawn there. Its really dark so pls fix bug fast. Sry for my bad english im german
I need help