which kit do you have loaded up?
It's a good question. I believe this is because the audio sample you have loaded is not able to be layered... I suspect should you load an EXS24 Sampler drum kit in where the samples have been multi-layered then it'd work fine.
Basically, when it comes to sampling a real drum kit, the sound of say a snare drum hit softly is very different to if it's hit harder, or very hard. So, when playing back sampled instruments, like our drum kit, you can get more realistic sounding drums from Logic (or any Sampler) by recording/sampling the sound at different velocities (or strength of how hard the snare drum is hit).
So, let's say we recorded a snare drum being hit 5 different velocities/strength(s)... . In the EXS24 sampler you can assign a velocity to each of these individual hits for the snare - so the snare is triggered by one note on your MIDI keyboard (let's say C2) - but if you hit the C2 note very softly it'll trigger playback of the "very soft snare" sample... hit it a bit louder... "softish snare"... etc, etc.
Anyway, so you import these sampler instruments into Ultrabeat - so I believe this control for layer and velocity affects multi-layered sampler instruments.
... well, I hope this is the answer... because if it's not I've really gone round the houses with my answer ;-)