No slimes spawning

There is a chance that I've just been extremely unlucky...

I've been running a snapshot server for a few weeks, and at the launch of it, when we were in 19b, I think, slimes were spawning in a swamp and everything was working well. However, fast forward a couple weeks to 21b, and it's impossible to find slimes. We've dug out a 3-high chunk (in a slime chunk) on both levels 16 and 5, with no slimes. We've also gone out into the swamps at night, with no luck. Again, I'm not sure if this is a bug or if I haven't been looking right. Either way, help would be appreciated.