Drum loops cutting out upon vocal addition

Hi all,
I am experiencing an odd drop out from EZdrummer whenever I start to add vocals to my song. Not all the drums disappear, but it seems to play only about a third of the notes programmed. None of the other 12 or so tracks are effected when I add the lead vocal track, but the drums are the only midi thing going on.
Any idea why? Some sort of odd trigger-gate being impacted by this new addition?
Thanks
- Jim