Be sure you're starting with a smooth surface that is sealed.

Use GARDZ prior to the skim coat to seal the wall and provide adhesion for the skim coating. After skim coat is dry and smoothed, re-apply GARDZ to seal it. GARDZ can be your primer unless you're using a pastel or dark finish coat.

Michael

First I would scuff or sand the walls with a block or pad. I have a small orbital sander that works great. And then wash them, simple green is a great cleaner, and doesn't leave any film that would cause problems for compounding or painting. Mixing is a must!!, if you're using a 5gal. pail, most people don't have a drill large enough to turn the big mixing paddle. A small paint mixer, turned by any drill will work for you. Over-mixing can cause air bubbles, if you've ever watched a PRO taper, he is constantly turning the compound on the hawk. This helps release the air from the mix. Also, try some thinner coats, they dry faster and you can re-coat sooner. From time to time, you will get some bubbles, but the second coat should fill them in. Good Luck

You should also be using the green bucket mud; it is heavyweight compound; lightweight compounds are mixed with air on purpose to make them lightweight.
Casey

Thanks for the suggestions.
I'm using all-purpose joint compound, and I've been applying thin layers. I think that the problem is that the painted walls won't allow the compound to adhere properly.

I'll try sanding/scuffing and then cleaning on one part, and Gardz on the other to see which works best.

Thanks again.

When buying joint compound, check for the buckets that say ~DUST FREE~, something new. They have found a way to make the dust heavy, it falls to the floor, without making a dust cloud.

The new dust-free mud sets up very soft, and will scratch and dent easily. It is fine for ceilings, where it will not take any rough handling. I will never use it at people-level, certainly bad for corners.
Casey