Very low spawn rates for hostile mobs in 1.7.4

I created my world in 1.7.2 version, with a reasonable spawn rates, but now I got 4 or 5 mobs at most. I built a slime farm, and I just get a slime every 90 minutes!!, just one. I play in hard, and my seed is -786364582380729972, and i try to reinstal minecraft a lot of times. I know that this issue is duplicated, but I can't find the solution, so please help me, or tell me that there is no solution for this but tell me something please, I'm so sad about that, PD: Sorry but my english is not really good, but I am from Spain