Colloquial form of もう一つ, used in casual speech to request or mention an additional item.
See also: もう一つ
Give me one more, please.
I have one more question.
Used with a negative verb to mean 'not quite' or 'not very', implying something is lacking or insufficient. Often appears as も一つ~ない.
See also: もう一つ
This movie wasn't very interesting.
I didn't quite understand his explanation.