While cleaning, did you unscrew the nozzle from the tip assembly? If so, did you make sure that it's fully seated when re-assembling? If it's not fully seated, it may be projecting too far, so when you re-attach the air cap, it may be closing the gap around the tip.
Also, check the nozzle for "flaring" damage. If the needle is pushed too hard into the nozzle, it may flare the tip of the nozzle. (This is less likely, but possible)
The threads on the air cap are rather fine and shouldn't be a possible air leak, but you can apply a little wax to the threads to ensure a good seal.