Best thing to do is remove the screws and take the brakes off. Underneath will be a plate ith 2 metal pins thats holds in down. Make sure these pins are flush, tap them to push them down. Now have a look at the two metal teath at the front of the brake. Make sure there straight and not bent. If there are bent you can carfully straighten them with some pilars.
Lastly cheach the 2 tabs where the brake teath gointo on the heel piece. Again make sure they stick straight up. When you put the brake back on, make sure the teath sit in these housings cleanly. Do up the bolt ensuring the teath are still in postion, and see if that helps at all.