Try shooting from behind the net more up the field, it will clean up the background. I would also just stick with 300mm and between the 5.6-6.3, in that range will give you the separation from the backgrounds. The content is there, action is good, so no problems there. Just keep at it, and give the end a try, you will have to wait for the action to come to you, but you also can't cover the whole field.