Tough Obstruction No-call

[turk]

I was coaching a LL All Star practice game last night. We only had a Plate Umpire, but otherwise, all rules were applicable.

R1, on first, gets the steal sign and takes off. The throw comes, and the shortstop sets up in the base path on 2 knees, almost bent over like you would in a leap frog game, and fields it on one hop, but is bobbling the ball. As she is bobbling it, R1 arrives, but has nowhere to run, because the fielder is in the path. Instead of jumping over her, or running around her, R1 slows down, the fielder gets control of the ball, and tags her.

PU makes no call. I consider kicking dirt on him and calling him nasty names in Spanish, but he is a good firend and I never yell at umpires. However, I cannot see how it is not obstruction. I know the runner has to concede the base path to a fielder making a play on a thrown or hit ball, but a fielder cannot block a base without possession. I want to coach the girls to be agressive on the bases, but do I tell them to run the fielder over in that scenario, go over them, or around them?

Guide me, o wise sages of fast pitch.