Since you don't indicate which model your using, it is difficult to nail it down (and everything might not be applicable to you), but here are some quick checks that I can think of:
The 1st obvious case: You might not have enough hit from available choices.
a) Did you specify the 0%miss constraint correctly? Remember the optimiser increases the main stat your optimising, so don't try to optimise miss chance directly, since you will end up missing a lot. Optimise dps, while having misses <= 0. If the optimiser
cannot reach the goal, it should report it as such, if it isn't complaining and still giving this result, something might not be specified correctly.
b) Are you using a melee or caster model? 1 handed or dual-wield? The amount of hit differs significantly? Or a tanking model? Expertise requirements etc. Is it possible to reach your model's cap? (Before we can do anything, we need details on your
c) Have you checked all buffs?
d) Have you selected the +hit type talents you have available?
e) Are hit gems/enchants available for selection?
f) Have you made sufficient gear choices available to the optimiser? Are they selected with only specific gemming options (blue diamonds) or general regemming (green diamonds)?
g) Are you looking at the correct miss chance result output?
If you have double checked all of these and it still doesn't work, there is the possibility of a bug. But from my own experience, most cases where I have encountered something like this, it turned out to be something I forgot to tick.