I've seen quite a few people. in various places, complaining they can't close the case of their Sansa Fuze V1 and 2 after replacing the battery, etc.
I've discovered the technique to successful case closure is to engage the front panel into the lower case by the top edges first.
Line the 2 top edges up, then apply a slight pressure to the bottom edge of the front panel to ensure the 2 clips on the top edge are fully engaged.
Then gently apply pressure down both side edges working from the top edge down.
All the edge clips should close nicely, and the case should feel nicely solid.
NB if replacing the battery, ensure the wires are tucked out of the way.