EZDrummer dropouts

Hi folks-
Using X1 producer, there are times my EZDrummer "forgets" to play many of the notes I have asked him to play. After I start the mixdown and applying vocal channels to the vocal bus and vintage channel to the bus that feeds my master (maybe an awful set-up, but I'm new to this), all of a sudden many of the drum hits that were programmed disappear... but not all and not in any pattern. It doesn't seem to matter what route I send the signal for that channel. This issue has popped up a few times for me.
Any ideas how to get him to play ALL of what I have asked of him?
- Jim