I'm down in the dumps = I am depressed, sad, or unhappy
- I broke up with him, now I'm down in the dumps.
- She's been down in the dumps since she quit her job.
I can put my finger on [it] = I can recall (or identify) it exactly
BUT this idiom is almost always formed with the negative:
- I can't put my finger on it, but there is something familiar about his face.
- This food has an interesting flavor, but I can't put my finger on it.
- There's something odd about that phrase, but I can't put my finger on it.