Remotely cancelled samples

Im fighting with MD for sending all chosen wav samples in it. But from time to time MD is cancelling the process.
Samples are small and fit the limit of 2,5 MB.
Sometimes c6 makes messages that some samples are not PCM or Float (U-law). Others are ok, but MD cant receive them all.
Im trying to guess, why.
What specs should have a wav sample to fit MD?
How to check multiple samples?
Or maybe the problem in c6 settings?