"I can't put my finger on it"
definition: We use this expression to say that we can't explain or fully understand something.
(nemůžu na to kápnout, nemůžu na to přijít)
A: She looks different, doesn't she?
B: Yes, but I can't put my finger on it. Has she changed her hair?
A: Why don't you like Robert?
B: I don't know. I can't put my finger on it. I just feel we can't trust him.