The word 今ひとつ has the surface meaning "yet another, one more", but it seems to be more popularly used to mean "not entirely satisfactory", for example:
その薬の効果は今ひとつだった。
The effectiveness of the drug was not up to scratch.
Where did this other meaning come from? Which meaning do speakers feel is the more primary sense? My guess is that it progressed from "one more" to "superfluous, unnecessary" to "not performing its duty".
At least in modern Japanese, いまひとつ no longer means "yet another". This phrase now always means "(not terrible, but) not up to snuff" or "not really (perfect)". デジタル大辞泉 has an example of "いまひとついかがですか", but this is something we might see only in older novels.
According to this article, the modern いまひとつ is a relatively recent expression that only came into use in the 1950's. At first, it was a negative polarity item that was always followed by 欠く, 足りない, 分からない, etc., indicating that only "one more piece" is missing for perfection (or full understanding, etc). Later, this started to be used without an explicit negation. So something like "one more piece" became an ordinary no-adjective meaning "not up to scratch".
There are similar expressions such as もうひとつ, いま一歩, もう一息. These tend to be used with some form of negation, but it's not a requirement. (Of course, もうひとつ also means "yet another".) いまいち emerged in the 1970's as a colloquial/slang version of いまひとつ, but it's now widely used in formal documents as well.
