Bubbles in your ceiling are an indication that the ceiling has lost adhesion to the substrate. Although it may be possible to scrap back the affected area and respray the ceiling it will most likely continue to bubble and peel loose. In fact depending on the cause of the problem it is very possible the entire ceiling will bubble or fall from the moisture when resprayed. The best solution for this problem is to remove the popcorn texture and start over by removing the existing acoustic and applying either fresh acoustic or wall texture.