As bunkerbarge has said we could do with seeing the cans.There is more than one reason for this to happen and Bunkerbarge has highlighted one of them,another is primer coats been applied to thick or too fast,what I mean by that is simply one coat after another and not enough time for the thinners to evaporate causing solvent trap,then you apply top coat thinners in this will attack primer and cause shrinkage which can look to the untrained eye as cracking.Another old fav is the fingers,yes those wonderfull things which harbour all kinds of nasties.One should ALLWAYS dea grease with a proprietary solvent or wipes and strictly speaking wear latex gloves,hey I admit I don't do that BUT I will not handle a model or whatever after it's been degreased.
Another reason for paint cracking is dissimilar products.
Lets us see what you have used
Stavros