Ok this is a slightly weird fault that I can't quite get my head around.

Almost every other shot the pistol will not fire. It feels as though the sears don't engage (trigger is spongy) This only seems to happen on more than 1 pump and strangely only when a pellet is inserted
The pistol will always fire if no pellet is inserted.

I've reset the trigger to its original state in case I had 'over' adjusted it but no joy. Opening and closing the action sometimes clears the fault, as does giving it an extra pump (again not every time). A sharp tap sometimes clears the fault to, but not something I'm happy to do other than to clear the gun for safety sake.

Before I send it off for repair (since it's under warranty still) any ideas if it may be something simple?

Is there an exploded diagram of one anywhere btw?