this was a problem I had with boston. she could care less if I was waving a million ropes over my head with balls attached to each one. she only wanted 1 ball and would not tug. also, once she got the ball she was in no hurry to run back. she came back, but she sauntered back with this look on her face that said, "look mom I got the ball" we tried having me run away from her as fast as I could, well she would chase me but it only worked a couple of times. I had to hide each ball after a heat because she wouldn't run another one if she saw I had a ball already.
what my trainer had me do was take away ALL the balls and toys. Boston was only allowed to tug for playtime and she only got to have the ball during flyball practice. do you have any idea what happens when you de-ball your house? OMG she wen't nuts. she wined and searched my entire house for a ball. now she can probably do search and rescue because this dog can find a ball buried under a mound of rubble.
now she tugs...but she still won't run back to me. I don't think she gets the concept that the race continues on the way back. lol she thinks it only a race to get the ball out of the box before the other dog gets it. *sigh* someday we'll be ready to actually compete. lol but flyball is a blast.