apps problems iPhone 3G

Hi,

I have an I phone 4 S;

sometimes i cant open my applications ( free downloaded from apple store).

Pressing on the app icon, for half second the screen turns to turn on the app but immediately comes back on the main menu and the app icon go down on the bottom bar. even from there i cant open it.

what can be? this is not happening all the time and not with all the apps.

thanks