What am i doing wrong? A MNK low output.

[KikoriL]

It'd help possibly point out what's being done wrong to know what you're currently doing. How you start, at what rate you (re-)apply (de-)buffs, and so on.

It's probably far from optimal, but what I end up doing looks generally like...
- Fist of Wind, full chakra bar.
- Shoulder Tackle from behind, Perfect Balance, Demolish, Riddle of Wind re-tackle while moving to the flank, Snap Punch, Twin Snakes, Dragon Kick, Riddle of Fire, Bootshine, Internal Release opener
- Follow immediately with Forbidden Chakra, GCD, Brotherhood, GCD, Diversion in Ex content but otherwise Elixir Field, GCD, Steel Peak, GCD, Howling Fist, GCD, while putting a priority on any earned Forbidden Chakra that pop.
- Prioritize Dragon Kick / Twin Snakes / Demolish only if their respective (de-)buffs are at 6s or less upon hitting their usable forms.

That's just me. Better MNK will probably correct me on a few points, but I still generally hit top two in the charts for any 8-man or more content, so it's stable enough.

Just remember that constantly reapplying (de-)buffs and skipping the higher damage counterparts to each form is wasteful. Especially in Demolish's case; just like healers falsely assume the heal over time makes re-casting spells like Medica II over and over better without letting the timers expire, reapplying DoTs before a certain point costs you output.