Mine has a little movement (side to side) when it's fully extended. There shouldn't however be a large gap at either end. If the space on your phone is larger than the spacing *between* the two pieces (looking at it from the side) there's the possibility that it's a manufacturing issue or worse yet - counterfeit parts/copy.

When extended the phone should not flex or bend. It's ok if it shifts (side to side) a little. Be careful - if you force it too much it will pop right off of the rails. :(