Jitsugen dekite nai

A scriptwriter in an interview describes an unused idea:

けっこう頭ひねったんですが、それは実現できてないですね

Does this mean they couldn't implement the idea, or just that they didn't? The first clause seems to indicate the former, but the できてない construction and use of present tense seem to indicate the latter to me. (Then again, this scriptwriter uses incorrect tenses a few other places in the interview.)

Thanks very much for any help.

Here 「い」 just before 「ない」 is omitted.

「そのアイデアを実現できていない」 may be translated like this.
: have not made the idea embodied.
: cannot have realized the idea (by now).

> Does this mean they couldn't implement the idea, or just that they didn't?

Those two refer simply to the past.

「ていない」 focuses on the present.
For example,
「宿題が終わっていない」 means
the current situation where the homework has not been finished.

> this scriptwriter uses incorrect tenses a few other places in the interview.

Between English and Japanese, in some cases,
different tenses (plus aspects) are chosen to mean the same.

e.g.
EN: John said he had not yet reserved our seats.
JP: ジョンは僕らの席をまだ予約していないと言った。

We say 「予約していなかった」 when we mention the fact in the past.

e.g.
EN: At that time, John had not yet reserved our seats.
JP: その時点では、 ジョンは僕らの席をまだ予約していなかった。

First of all, it's natural to assume that the subject of the sentence is either "I" or "we".

So, if it is the scriptwriter saying the sentence, it means, "I, the scriptwriter (or we as a team) thought about it a lot, but haven't been able to make it happen."

Does this mean they couldn't implement the idea, or just that they didn't?

The scriptwriter doesn't say whether (s)he deliberately avoided the implementation, or something forced him/her from implementing. The reason (s)he says "not been able to" could be just because (s)he regrets not showing the idea to his/her future audience.

The first clause seems to indicate the former, but the できてない construction and use of present tense seem to indicate the latter to me. (Then again, this scriptwriter uses incorrect tenses a few other places in the interview.)

I'm not sure if I understand what you're trying to say, but for the record, the sentence is not incorrect in terms of tense. (S)he's saying that (s)he worked on it in the "past", but "still today" hasn't been able to put it to practice yet. But because of this, the sentence does give us the impression that (s)he sort of ran out of ways to make it happen, even though (s)he still hopes to make it happen.

Anyway, it's a very common way of saying things.

I hope this answers your question.