we had a young herd bull semen tested

We've had a group of heifers in with 2 young herd bulls. The heifers are now calving and just by the looks of the calves, they are almost all out of one bull (home raised). The better bull (purchased) seems to have only bred a few of them, so we had him semen tested. The first time, not much of anything showed up under the microscope. So the vet tested him again. This time the report was that he was"real good". Is it possible that a bull has good semen but it isn't getting transferred into the cow? I'm just wondering because it took two times of testing him to get any semen.

Some bulls don't respond well. I had a bull for 10 years, semen tested him every year never able to collect a sample. Sent him to town at 12 and never missed a cow. Always made me nervous. If you've seen him breed I wouldn't be worried. I always watch my yearlings get one confirmed breeding before I get comfortable. One time bought a bull, he jump ejaculate and back down. Never got it in too early too the party you could say. Gave up after giving him dozens of opportunity.

Attitude tends to be another reason for many people I know.  Heifers have ovaries and well enough said, lol.

If you think some of the calves are out of him and he tested good I would attribute the fact that he didn't breed as many heifers to a couple different things.  If you run more than one bull in a pasture there will be a more dominant bull that will breed a majority just like there is a boss cow in the bunch that runs the show.  His libido could be lower than the other bull as well.  If he is the better one and you want him to breed more heifers I would put them in two separate groups at least for the first part of the breeding season if possible.