I've got a theory on this... Occasionally when i plastered i would get the air bubbles coming through and it was a mystery. last summer i did some plastering for my dad and got him to mix it up with me, he was on the drill and kept lifting it in and out of the mix... not the way i do it but i left him to it. started putting the mix on the ceiling and had shocking probs with the bubbles. next mix told him to leave paddle in the mix as he stirred, and a perfect mix going onto exactly same surface, no bubbles at all.
cocnclusive evidence, its air getting mixed in with the plaster so keep the paddle low, ( a taller, narrower bucket makes this easier). If you do get the bubbles there is very little you can do about them. don't waste time trying to get rid of them at the start, focus in the rest of the wall and have a go with a sponge near the end.