Learn Russian: Сериал не то чтобы затянутый, а просто не очень динамичный. - The series is not exactly drawn out, but simply not very dynamic.

The phrase не то чтобы mitigates or softens the statement. It translates to "not exactly" or "it's not that..." Instead of a flat denial like Сериал не затянутый ("The series is not drawn out"), it concedes that the series might feel a bit slow, but then corrects the exact description in the second half using а (but rather).

Why do we use the conjunction а here instead of но for "but"?

In Russian, а is the standard conjunction when you are correcting a negative statement with a replacing alternative (the "not X, but rather Y" structure). Since the first half negates затянутый, the second half provides the correct alternative description (просто не очень динамичный) using а.

Where does the word затянутый come from?

It is a past passive participle formed from the perfective verb затянуть (to pull tight, to drag out or delay). Here, it acts as a standard adjective meaning "drawn out." It is a very common vocabulary word for describing movies, books, or TV shows that feel too long or poorly paced.

Could we use the short-form adjectives (затянут, динамичен) here instead?

You could grammatically, but it would sound much more formal or literary. Using the long-form adjectives (затянутый, динамичный) is much more natural for expressing a subjective, conversational opinion about a TV show. It frames these qualities as permanent, defining characteristics of the series rather than temporary states.