Unfortunately a battle that I experienced with friends when we used to spend many hours in the 1970's painting Airfix Napoleonic Armies before enjoying mammoth historical battles. You always found that even with the greatest care and having glued units to card bases, paint would fall off. It was the Polythene they moulded in that was so flexible living in Cyprus even expansion due to heat or shrinkage as the air cooled was enough to break the tiny grip that the paint layers had. You could wash the figures in strong detergent when brand new and it made not a jot of difference.
The only cure was to ask for the hard styrene or white metal figures to be bought for Christmas and Birthdays by mail order from the likes of Military Modelling or Airfix monthly magazines and slowly oh so slowly retire the polythene figures to airgun, fire or dustbin.
Memories of happiness followed by sheer frustration.
John