The parliamentary debate would seem to have been with the intention of permitting actions in Syria should the need arise. The raids hours after that vote was no doubt to demonstrate capability and intent.
The subsequent inaction could have several reasons other than a lack of suitable targets.
One might be to leave the Syrian TOO to the Russians. Another might be to concentrate in Iraq to try and clear that rats nest first. Either shows concentration and maintenance of an aim rather than a fragmentary effort in the whole TOO.