When i click an item, it won't pick up, but when i drag it will. Not in touchscreen mode!

When i click an item, it won't pick up. but when i drag it will, however it ocasionaly drops one of the items (if stacked) in an open slot if i move my mouse over it, or it switches places with the item in the slot if full. I don't know if it's an update or something but it's driving me crazy.