one more; another one
Literal adverbial use meaning 'one more' or 'another'. In modern everyday speech, もう一つ is usually more natural; 今ひとつ here has a somewhat written or deliberate feel.
There is one more point I would like to confirm.
If we give one more example, this difference is easier to understand.
not quite; not entirely; somewhat lacking
Common evaluative use for something that is almost acceptable but falls short. Often appears with negatives or mildly critical expressions such as 今ひとつ分からない, 今ひとつ足りない, or 今ひとつだ.
Even after hearing the explanation, I still do not quite understand.
This dish is not bad, but the flavor is not quite there.
I don't quite understand his explanation.
The food at this restaurant isn't quite up to par.
This proposal is somewhat lacking in persuasiveness.