Yamaha DTXpress/DTXplorer/DTXtreme

I've bought DTXpress II. It's amazing, but...

I've noticed something strange, which really bothers me, but I 
haven't had the time to read the manual. I really really hope that 
this can be fixed...

When I'm hitting snare and hihat pad at the same time, they don't 
respond. It's either snare or hihat, not both, like they should. This 
can be even seen on lcd - one of the pad not responding when the 
other is playing. Every other combination (tom-snare, hihat-tom, 
crash-tom...) works just fine. I've even changed the pads. The same 
problem (snare-hihat combination is problematic). But when I'm 
hitting snare and hihat pad very hard, then they both start to 
respond at the same time. What can be done? Is this maybe like a 
general issue with DTXpress or is it just on mine? 

Damn, did I explain this right?

Any help would be greatly appreciated!

Thanks