Yamaha DTXpress/DTXplorer/DTXtreme

My son got a new DTXpress for Christmas and he loves it.  We're still 
learning the ins and outs of tweaking the thing but we've been able 
to figure out most of it.

However, we can't get the rim (snare or ride) to work consistently.  
Sometimes we'll be able to get it to sound but a lot of times we'll 
hit it and only get the pad sound (or nothing at all).  Maybe we're 
not hitting it right.  Is there a sweet spot?  Are there adjustments 
that someone might recommend?

I'd appreciate any advice you can offer.