Tagging on a bobbled pop fly

[jpattersondc]

I play on a rec league co-ed team. Runner on third, fly ball to left field, left fielder "catches" ball in his glove, runner on third tags and advances home, however left fielder bobbles the ball while transferring the ball to throw it but does not drop it. Umpire calls runner on third out for not tagging up. What is the ruling on this? Is the runner at the disadvantage for having to wait for a clean catch, or can the runner tag when contact is made?

Jason Patterson, Folsom, Ca