Quote from Dicemage
I found one small slime after two weeks of playing. I killed it and get nothing and now that same cave keeps empty. Am I doing something wrong? I filled that place with torches but if I understood right they are friendly so it should not affect of their spawn.
Don't kill them with a weapon! If you do that, they will drop nothing. You have to punch them, or use a very weak item, to kill them. Then they will subdivide, and the little ones will drop slimeballs.