Pack version 1.70c

Slime spawns on slime islands - whether in the overworld sky or on the Nether lava ocean - simply do not happen. It is remotely possible that I've just been unlucky, but slime spawns were very common in past versions of the pack, now exploration of about 20 slime islands yielded not a single slime spawned.

Thank you for the report, I will investigate the issue.
This could be caused by updating TiC, but I can't remember if I did that atm ^^'

I am playing on 1.72 and my slime islands are not spawning slimes either. My slime island actually had 2 normal trees in addition to the slime trees, and there were 4 cows up there somehow...

Silly question, but you're not in peaceful mode right?

No, I was in Hard mode

+1, can confirm - all islands empty (on 1.72)

Update: On a newly created world in 1.72, there are slime mobs on these islands.

But not old worlds?
Are other mobs spawning? You could have reached the hostile mob cap, that's reached pretty quickly in E2E.

If this helps, I checked in save game started in 1.69 now updated to 1.72 and found if I fly around and find a new undiscovered slime islands and fly straight to it and land on it, either no slimes have spawned or sometimes there are other entities up there (eg: deer, cows found so far) -
however if I find a new island and stop around 20-30 blocks away and wait for around 30ish seconds, blue slimes will spawn.

Yes, can confirm that new chunks will spawn slime - the ones near my base remain empty though, even after minutes of waiting around. But I guess that's good enough? At least good enough to build a mob farm as suggested in #1424. So I think this can be closed.