Yamaha DTXpress/DTXplorer/DTXtreme

I see alot of people posting this problem but didn't see anyone with 
a real indication as to how to fix it.  My kick pad is to the point 
that 1/3 to 1/2 of my hits are not coming through.  Occasionally I 
hear a faint, low volume hit, but not what it should sound like.  
This is becoming increasingly frustrating as I am finally to the 
point that the rapid fire is coming to me quite easily and I can't 
hear it.  Any thoughts?????

BYW - I'm playing through headphones, w/ and w/out cd accompaniment.