It is common when triggering a drum loop to use a MIDI Note Length that corresponds to the Sample Length. For example, using a whole note to trigger a 1 bar drum loop. The problem with this is that the Note Off and the next Note On (which are on the same key) can happen at the same time and cancel each other out. This results in notes that get cut off or don't play.The best way to resolve this is to shorten the MIDI Note, and either open the Gate (if present) or increase the Amplitude Envelope's Release Time in the sampler. This will allow the sample to play out even after a Note Off has been sent. Also, depending on the sampler, retriggering the sample should cut off the previous one (this might be controlled with an 'overlap' parameter).