I have recently painted a couple of rooms in my house, straight onto the plaster. I have given then a couple of coats and left plenty of time between. In both rooms the paint cracks away from the wall around the radiators. Any ideas please? I have also noticed in another room (that I haven't paited) that this has also happened!

Thanks

Were these walls previously painted? If they were it might have been an oil based paint that would need a primer before painting with a latex paint. If they were shiny they needed deglossed prior to that primer.

I assume these walls were washed before painting.

And if very old plaster (not drywall, or skimcoated plaster), it might have been painted with calcimine, which is a dull chalky white, and most paint can't go right over it. Or, if new or old and that area had never been primed, that could cause this too. So without knowing about your walls it's hard to advise.