Slime Chunks not spawning slimes

I used a slime chunk finder, and after I dug out the chunk, no slimes spawned. So I changed my version to 1.7.4, and slimes spawned. So it wasn't the incorrect chunk, slimes just aren't spawning in their chunks.
EDIT: OK, well I tested further and they do spawn, just very rarely. I've seen two slimes spawn, total.