As Anova said, because of latency the server position for the mob is a little bit ahead of what you see. If a mob is in the middle of turning it can be nearly impossible to judge where the correct position is for the server. I've also had issues where I don't get the bonus because the mob turns immediately after I attack (after on my screen, but it must have been before on the server). There are a few bosses that move a lot of their own will, but usually a tank should be holding things relatively still for you.

Quote Originally Posted by NargaHuntress View Post
... like they BARELY crit for me when i use the flabk attacks.
There aren't any critical hit bonuses associated with flank attacks, while bootshine will crit 100% of the time from the rear (in opo-opo stance). It would be extremely unlikely that you'd crit just as often with your flank attacks as your rear attacks. Also, you can get a critical hit from teh wrong position. The correct position for any (nearly all) of MNK's attacks only increases the potency.