I agree with you that 2.8 was too 'open' - particularly if you're shooting long. Recall that the longer you shoot, the thinner the slice of DOF you're getting for an equivalent aperture. This isn't to say that you should NEVER shoot sports that open--but if you do you have to be on your game, and, well, you'll have to accept that 4 out of 5 shots might be misses.

In addition to adding some shots to the mix, I'd like to know your camera, lens you were using, what focus mode you were using (point ? zone ? auto select ? servo ?).

Without seeing anything I'd suggest that it's possible that you were attempting to focus on subjects that didn't have as much contrast as the sunlight hitting the grass + accompanying shadows that might have been present. But it sounds like you were on the sun side (because of your shutter speed) - so, well, that's less likely.