Sounds like he's experienced. I'm just saying when you preg check 37 days after breeding, you are trying to feel something about 3/4 inch long floating around in fluid. Just harder, but can be done.
The other issue is that if she's not pregnant, why are you not seeing heat signs? Unless something is wrong, or she recently lost the pregnancy.
You're going to have to do something if you continue to not see heats. Maybe get the vet out again and do another preg check, and if she's for sure not pregnant, ask why no signs of heat.
I assume you are experienced at looking for signs of heat. Of course, if there are other cows, it's easier. Remember the one doing the riding isn't necessarily in heat. At some point the one in heat has to stand still while others mount her.
Also look for mucus discharge, bawling, and maybe blood on the rear a couple days after ovulation.
Here's a good article I was reading recently.
http://www.wikihow.com/Tell-when-a-C...r-is-in-Estrus