not much information. is it the drywall paper bubbling? or the mud? if you're sanding between coats? you must wipe all dust off! it will draw into the fresh mud and pull away from dry coat. causing air to form= bubbles. if its the paper bubbling? could be faulty drywall=not enough glue when rolled. I once had an entire house 240 12' sheets faulty, when painted the paper bubbled all over once wet with paint. ended up putting two layers over whole house.
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