I'm assuming the Behr paint is latex or latex-acrylic. It's good stuff, by the way.
Some paints will dry, but take several days to actually harden up completely.
For example, some polyurethane based paints can take up to 10 days to fully harden even though they are tack-free within a few hours. You can actually dig in a fingernail and the paint feels soft even though it isn't sticky.
Latex and latex-acrylics aren't that bad, but if it's humid (and it is here!) it may take a couple of days to fully dry out, especially if you are using a brush or a roller that lays on a thick coat.
I know it's hard to be patient, but I'd recommend giving it several days to dry well in between coats. If possible, leave it in the sunlight for several hours at a time.
No problem sanding the primer and in-between final coats.....I do it all the time.....down to 400 grit. Give it an excellent, soft feel.
If you are using a flat paint, I'd recommend thinning it out a little with water. In particular, the Behr paint is pretty thick as they try to give you a "one coat" paint. Yeah, it's easy, but doesn't usually yield the result you desire...especially if you are going to the trouble to sand between coats.
If you want that professionally finished look, you can't beat a spray finish. I spray everything that I paint now. A $70 air compressor and a $20 HVLP spray gun work fine for small projects. Nothing else looks as good. It takes practice...don't expect to be an expert the first time. But I had a buddy spray his boat without any prior experience and it turned out awesome. He followed my advice and painted several practice scrap pieces of wood until he got the spray gun set just right and the paint thinned just right. In particular, the paint viscosity is key.