Welcome to the site.. and your very first question is a doozy! I'm just going to shoot from the hip and toss out ideas. the biggest is because you don't say who's heads.. only aftermarket.
Edelbrocks I'd probably trust right out of the box and a few others.. if you got these from ebay or similar, I'd say send them to a shop that does valve work and have them inspect before even installing.

And your results so far warrant that imho. Also, are you certain the head gaskets were installed correctly? They should've been marked to show top front or some such. If you are correct and the cylinders looked good, then the valves or gaskets are not sealing...