Sports shooting help (D90)

I just started doing sports stuff, as you can see because my daughter was playing soccer (or football as it's known most places). I'm missing some shots, but I'm not really sure why. I'm sure there's a technique element, but maybe my settings are not right, either.

I'm using D90 + 18-105 in Aperture priority, mostly f/7.1 today.
Handheld with VR on. AF-A and Dynamic Area mode.

After RTFM, I now believe I should have switched AF mode to AF-C and Auto Area mode, but I'd appreciate confirmation that this user-error is a factor. Also, any technique tips would be welcome.

I got plenty of keepers, but I really wanted to get #3, and the moment doesn't come back. But I'm willing to learn for next time. Thanks in advance!

Sample shots:

A stationary subject works fine.

Some movement, also OK--maybe because some players aren't really moving?

But would changing my settings help to get these shots right?