Also - make sure you thoroughly wash the inside of the shell with some hotish water and washing up liquid as any grease from your fingers will spoil things and stop the paint adhering.
I have in the past also dulled the inside of areas of the shell with abrasive (brillo or scouring pad - but be careful to consistently dull it...) this 'keys' the inside of the shell and helps the paint have something to grab onto as opposed to a perfectly smooth surface - although I don't bother as much these days.
The Solvent based rattle tins should 'eat into the shell' as I used them a lot before going to water based and once they are on - they are ON - so you must be putting too much paint down - patience is key
