Here is a bunch of questiosn I got in my last play.
1) The Detained condition said : The only action the Investigator can do is: INFLUENCE: Discard this condition.
If I fail my INFLUENCE test during my first action, do I lose my second action or can I try a second time ?
You can't do the same action more than once a turn, so in this case you lose your second action.
2) Norman Withers, the Astrologer can once per turn spend a sanity instead of spending a clue.
To use this special ability does he need to have a clue or not ?
No he doesn't.
3) Some mysteries allows to 'spend the clue you gained on this mystery'. How does this interact with the Astrologist ability. Can we put a Sanity token on the mystery and count them as clue to resolve it ?
No you need to use a clue gained from the encounter you've just had, the sanity token will not have been gained in this way.