It can be attempted over and over. Unless you optimize (Never Outnumbered skill trick, Imperious Command feat, Zhentarim Fighter sub levels etc.), that'd be a waste of your standard actions, though.
The notes in the "Try Again" entry for the skill seem to indicate that the defender gets a bonus to successive attempts, but no such modifier is listed.
Is this just up to the GM?
It blatanty says there, "...retries usually do not work."