When you use single shot focus... once the cam achieve AF lock, the shutter will release and continue until you let go the shutter button... it will not care if the subject moves out of focus in between frames.
When you use AI focus, once the cam achieve AF lock, the shutter will release and continue until you let go the shutter button. However the cam will re-confirm the focus and subject's vector after every frame. You can still achieve 5fps if your subject is either stationary or constantly moving.
When you use AI servo. First shot will fire regardless AF lock or not, subsequent shots AF must lock on before shutter can release.
So in your situation, it could be that the AF cannot be lock on subject.