Sorry for reviving this thread but please advise if you have resolved this. We're also experiencing this kind of behavior (Acrobat Pro XI and Windows XP)
If Acrobat is not yet opened and we executed the droplet, Acrobat opens and loads but the preflight action is not executed (we waited for a long time thinking Acrobat is still warming up but nothing really happens), we tried executing preflight droplet again (with the opened faulty Acrobat) and still does nothing. We needed to close Acrobat and reopen manually before we execute the preflight droplet for it to work again. Can anyone from Adobe check if this is a known issue?