There was an issue, but she couldn't put her finger on it.

put 在这里是动词，意思是"放置"。 her 是代词，指代"她的"。 finger 是名词，指"手指"。 on 是介词，表示"在...上"。 put her finger on 是一个固定短语，字面意思是"把手指放在...上"，引申为"确切指出"或"准确描述"某个问题或情况。