Hi Pa, I'm no expert in photography but being South African I do know a bit about rugby. What I can say, is that I think you'll have to up your shutter speed for these kind of action shots. Even if you have a bit of low light, the higher ISO capabilities of the K-x should still allow you get you good shots. At f5.6, your DOF for such shots might also not be ideal. I also tried using the drive mode continuous shooting the other day at my son's cricket game, and you might try that. I use AF-C with spot focus, normally Tv mode (as there is lots of movement I believe shutter speed to be the priority), shutter speed about 1/500, Auto ISO set between 100-3600.
If you had the drive mode on in the third shot, you might have got a shot of the players contesting for the ball in the air, which would have made it more interesting in my opinion.
If you don't know rugby, the shots will most probably not "speak" to you, although the third shot tells me a different story. I tells me, " o what the hell, lets have a rest and leave them doing the hard work"