Yamaha DTXpress/DTXplorer/DTXtreme

Hi

I have just purcahsed a Yamaha DTEXPRESS (not II or III) electronic 
drum kit.  The kit is 2nd hand but looks new and has hardly been 
used by the previous owner.

The kit sound fine however:-

When I hit 2 or more pads at the same time very often one pad will 
not sound. 

Even when playing simple beats the snare or hi-hat sometime does not 
sound when they are hit at exactly the same time.

Any help here would be greatly appreciated.  Is there a simple 
setting I can change on the brain?

Many thanks