Why aren't any slimes spawning in my slime farm?

Solution 1:

Have you ever seen a slime in your farm? If answer is no, good chance you got something wrong somewhere in your calculations.

If you have seen a slime, then very likely there are just too many other places for MOBS to spawn in the 150x150 region around you, that fill up all the MOB slots. Time to break out the torches, and start lighting everything around you.

What I have found best, is to just let it sit for a long time while I do something else in the region, then revisit occasionally. This helps, because slimes spawn more slowly than other MOBS, but they do not despawn like other MOBS, so if you have a good slime spawning region, they will continuously accumulate.... It can get crazy.

Solution 2:

According to the Minecraft Wiki:

Slimes spawn only 1/10th as often as other hostile mobs, so they can be hard to find.

So, according to the information you have given, you are doing everything correctly, and Notch is simply not smiling down upon you. One thing that may increase you luck is lighting up the slime farm in some manner. Slimes do not have lighting restrictions so preventing other mobs from spawning will increase the chances of a successful slime spawn.