In one reference, it is said that this expression simply means that something does not meet a given standard. However, another goes on to say that this expression means "not at all." What is the full picture?
Verb-root지_못하다 is the "long form" inability form...It means the speaker is unable to do the verb..it is not possible. It is rarely used with DVs (descriptive verbs or "adjectives") but in those rare cases when it is, the nuance of "dissatisfaction" is usually included.
For completeness, the two short forms are: 못_Verb and for Chinese-noun-based 하다 verbs (such as 주장하다), N_못_하다 (e.g. 주장 못 하다).
(Underscores above indicate "required" spacing, which is sometimes left out.)