wet sanding

you want enough water so u dont cut into the the paint,  enough to get out orange peel and dust, so use plenty of water

maybe try using a different primer, i have heard that some paints dont adhere to some primers well,  also  between the first coat and second coat, how long are u waiting.

I personally like to use as much water as possible.

As to how to know when  you're done sanding:  When the sand paper slides smoothly over the piece you're working on that means it's sanded to that grit.  To get it smoother go to a higher grit.

It sounds like your paint and primer arn't bonding properly.  I would suggest that you make sure to use primer from the same company that makes the paint that you're using. 

If you definitely are sure that you used the same two paint types on top of the same primer on another box successfully... then the reason your getting "peeling" or whatever is because you're timing between coats.

Normally if you let a recently applied coat of paint cure for more than an hour, you should not paint another coat on top of it until it completely cures, like 24 to 48 hrs later.

Try waiting 5 to 10 minutes between each coat, including when you go from on type of paint to the other.  Also, try not to put on thick coats... thin coats, applied enough, will eventually give you the coverage you need.