Another; one more — used to request or indicate an additional item or instance.
もう一つケーキを食べてもいいですか？
May I have one more piece of cake?
この箱にもう一つりんごを入れてください。
Please put one more apple into this box.
会議で重要な点がもう一つ残っています。
There is one more important point remaining in the meeting.
Even more; additionally — used to indicate an increase or strengthening of something (similar to 'more').
このプランは良いが、もう一つ詳しい説明が欲しい。
This plan is good, but I'd like an even more detailed explanation.
成功するには、もう一つ努力が必要だ。
To succeed, you need to make even more effort.
(Not) quite; (not) very; lacking — expresses that something falls a little short of expectations.
この映画は面白かったけれど、もう一つだった。
The movie was interesting, but it was not quite there.
料理の味はいいが、塩気がもう一つ足りない。
The dish tastes good, but it lacks a bit of salt.
彼の提案は悪くないが、説得力がもう一つだ。
His proposal isn't bad, but it's not quite persuasive enough.