Used as a noun phrase meaning an additional item or instance. Often followed by a noun or used alone.
もう一つください。
One more, please.
もう一つの問題があります。
There is another problem.
Adverbial use meaning 'to an even greater degree'. Often used with adjectives or verbs to indicate a higher level than before.
今日は昨日よりもう一つ寒い。
Today is even colder than yesterday.
Used in negative contexts to express that something is not quite satisfactory or falls short of expectations. Often appears as もう一つ～ない or もう一つだ.
See also: 今ひとつ
彼の説明で、問題がもう一つ複雑になった。
His explanation made the problem even more complicated.
This dish is not quite flavorful enough.
彼の提案はもう一つだ。
His proposal is lacking something.
Variant using numeral 1; common in informal writing.
Combination of もう (already, more) and 一つ (one thing). The adverbial and negative senses developed from the idea of 'one more thing' needed to reach a satisfactory state.