Focus issues any help?

I was having a heck of a time yesterday with this lens and Im lost being a newb.

I was shooting with a monopod locked into a fence so very stable. I would then push the shutter release 1/2 way to focus,then shoot as the target moved. Some were even stills and they were out of focus, almost like the camera was grabbing the wrong subject yet there was no hunting.
Any ideas or help would be appreciated.

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I can't tell on the first shot, but the second shot has focused much much closer to the guy in the background - or more likely the fence. The pitcher's foot is not moving (so no motion blur) and it's definitely out of focus. Yet the "Kelley" on the box behind the fence is pretty darn sharp.

Checking again, the poster on the left in pic #1 looks sharper than the player. Another mis-focus to the rear??

Are you using the center focusing point only? This will ensure that you are focusing on the intended subject. I use center-only about 99% of the time, at least. IMHO it the way to go for action.

Yes I am using center focal point only. I have several other shots, infact last night I took 85 shots. The ones after dark had set in were a bit better but still could not get the focus correct. It seemed if I set the exposure to be bright enough that it would not focus no matter what shutter speed I used.

Now I did shoot a police car earlier on before the game, it was traveling about 50mph at a 45* angle toward me and the pictures came out smooth as silk I could even see his badge.....Seems like the human target is the issue as everything else I shoot turns out pretty good.