check if any error is there or not . by pressing f9.
if any error is there try to rectify it. or if no error is there.
then go to,
System->Repair->Repair JS files.
System->Repair->Rebuild Js Grouping Files.and at last.
Ststem->Repair->Quick Repair and Rebuild.
and one more issue, if ur running this on server, then do one thing give permission the root folder. and repair it will work . because in sever it happens sometime.
most of the cases, just give permission the root folder, clear the cache, and repair. it works fine.