Not put your finger on something
Definition 1 of 1
Pro Tip 1/3Intuition Involved
The phrase often refers to a gut feeling or something you sense but can't explain. 
She was friendly, but I couldn't put my finger on why I felt uneasy.
Pro Tip 2/3Hint of Uncertainty
Use when there's a lack of certainty or when feeling unsure about the exact reason or cause. 
The story is good, but I can’t put my finger on why it doesn’t excite me.
Pro Tip 3/3Subtle Awareness
Ideal for expressing awareness of subtle aspects that are not immediately obvious or visible. 
The room was the same, yet I couldn’t put my finger on what had changed.