Not put your finger on something - Idioms by The Free Dictionary
not put your finger on something
not put (one's) finger on (something)
To be unable to recall fully, clearly, or correctly. I can't put my finger on what this is reminding me of. There's something I was supposed to do this afternoon, but I can't put my finger on it.
not put your ˈfinger on something (informal) not be able to say exactly what is wrong or different about a particular situation: I knew something she had said wasn’t true, but I couldn’t quite put my finger on it. ♢ There’s something wrong with these statistics but I just can’t put my finger on what it is.