Walls and Ceilings - bubbling joint compound

I am skim coating my walls with joint compound because I removed wallpaper and the walls are a mess. I scrubbed the walls clean to remove anything left behind. The problem is, when I skim coat the walls with joint compound (already pre mixed from manufacturer) in several different areas the compound bubbles. I waited for it to dry then sanded the area and washed it. I then recoated it but the same problem happened again. Is there a reason this is happening and what can I do to correct this problem? Thanks for any help.

Bubbles in the mud over tape are where the tape was not bedded properly. Not enough mud was under the tape or the tape was pressed down so hard it forced the mud out. Such a place must be removed and replaced.

Bubbles like pits in the surface are air in the mud. Better mixing seems to help. Adding a dash of dishwashing detergent to the final mixture is used to make a smoother final film.

If the mud won't stick to the wall, all the wallpaper glue and other contaminants are not removed. Thorough cleaning in wallpaper removal is vital to successful covering later.