Discontent or discontented could easily fit.
Here is a list of several words that are somewhat synonyms to this concept, and depending on context might prove useful:
blue, complaining, crabby, disgruntled, displeased, disquieted, exasperated, fed up, fretful, griping, perturbed, picky, restless, ungratified, upset, vexed
It depends partly on how active the discontentment is. Exasperated, griping, crabby, etc, have connotations of outward display of discontentment, while blue, discontent, disgruntled, etc can be relatively contained.
Restless presents an important nuance: it does not have to be negative. This is a valuable word to use when the discontentment is not a bad thing, especially when there is hope of resolving the cause of restlessness.
Perfectionist might be a helpful word (also maybe stickler or idealist); this describes someone who can't relax until the thing he/she is working on is perfect.