Her only problem is a lack of confidence.
她唯一的問題是缺乏自信。
Lack of sleep had made him irritable.
睡眠不足令他煩躁易怒。
If he fails it won't be for/through lack of effort (= he has certainly tried).
他若失敗了,那肯定不是因為他努力不夠。
We won't be going away this year - lack of funds, I'm afraid.
今年我們恐怕不會去度假了——因為沒錢。