Kosina Zoltán

Same here. 4 forts, 3 wither skeletons.

It seems due to a bug (most likely MCPE-21856) slimes has so low spawn rates, that they only spawn if almost no other mob spawns around them.
And something seems to be also wrong with the light level check. I was able to spawn slimes only on minimum (0) and maximum (15) light level. On the minimum light level, other hostile mobs made slime spawning very rare. On maximum light level, on daylight, they spawned just fine together with horses, and underground on glowstone they also spawned. Torches with light level 14 seemed to be not enough for them.

So it seems both the spawn rates and the light check is broken.