1. She wasn't impressed
2. She was unimpressed
3. She impressed them.
4. *She unimpressed them.
I propose that 1 and 2 contain verbs in passive form with a null subject. 3 contains a verb in active form. 4 is ungrammatical because english doesn't have the negative counterpart of the verb impress. You can verify this:
1a. She wasn't very impressed (by the show)
1b. The show didn't impress her.
2a. She was totally unimpressed (by the show)
2b. *The show totally unimpressed her.
Note the oddity that verbs usually don't take very:
3a. *She very impressed them.
3b. She easily/quickly/totally impressed them.
3b is okay with an adverb but not with an intensifier.
The adjectival counterpart is "impressive" and the adverbial counterpart "impressively":
4. The impressive performance impressed her.
5. The impressively performed show impressed her.
Impressed belongs to a special class of known as "amuse" verbs that allow null subjects in their passive construction. In the process they take on some properties of adjectives. They are not adjectives however because they need to be verbs in order to be passivised. Take for example a pure adjective:
6a. She was happy.
6b. Happy was she.
6c. *Happy was she by the show
The verbs in this class consist of words like: amuse, enchant, mesmerise, hypnotised etc. A non-exhaustive list can be found here in Joanis' paper:
[link]
Hope this helps