Monk Not Always Proccing From Behind?

[Niwashi]

I seem to position myself as best as I can, but I don't always seem to proc the bonus effects of the skills? Is it a chance thing, or is it me just not being positioned well enough?
Also, are the effects added if you START the skill at the opponent's flank, or FINISH the skill at the opponent's flank?

If you get the positional, you get the bonus (critical or higher potency, depending on the skill) for that positional. It's not chance. And it's based on when the skill completes and takes effect.

It's based, however, on where the server says the mob is facing when the server sees your skill take effect. There is always a slight lag time (magnified when you're experiencing enough lag to be noticeable) between when you hit a skill and when that reaches the server, as well as lag between when the server decides a mob is turning and when your screen will display that turn. So sometimes it might look on your screen like you completed an attack from the right direction, but the server thinks the mob had already turned a different direction by then so you miss it. That's the most likely cause of what seems like randomly failing it, particularly if the mob is in motion or turning around the time you used the skill.

The effects from the monk skills are not tied to the positionals, they are tied to the form. Only the effect from Bootshine requires an additional rear positional.

The attacks work without a positional, but several do more damage with one:

Bootshine: Opo-opo Form Bonus: Critical damage if dealt from behind target.
True Strike: Delivers an attack with a potency of 150. 190 when delivered from behind target.
Snap Punch: Delivers an attack with a potency of 140. 180 when delivered from a target's flank.
Twin Snakes: Delivers an attack with a potency of 100. 140 when delivered from a target's flank.
Demolish: Delivers damage over time with a potency of 30. 70 when delivered from behind target. Duration: 18s

Both the critical bonus for Bootshine and the higher potency bonuses of the others there mean higher damage. (Though the only way to spot the potency changes is analyzing the numbers. There's no direct visible indication that there was X points of basic damage + Y points of additional bonus damage. All you see is how much damage you caused either way.)