Thank you for your question to eXtension.

It sounds to me that the skunk has learned what the trap is, and is avoiding it. Certain animals develop "trap shyness," especially if they see others of their family trapped and removed.

I would try thoroughly washing and sanitizing the trap you are currently using, to remove any scent traces of the other skunks. Once the trap is washed, wear gloves when handling it, so as not to leave human scent on it, either. Since the skunk has become familiar with the trap being in the location you have used to capture its siblings, try moving the trap further away from the den opening, or somewhere nearby but not in the exact same place.

You may also want to consider buying or borrowing a completely different style of trap.  If the skunk recognizes your current trap for what it is, try getting a different brand of trap. 

This Nebraska article provides a bit of useful information for skunk control, which may prove to be helpful: http://ianrpubs.unl.edu/live/g1769/build/g1769.pdf . This Wisconsin article may also be useful; it has some good illustrations for blocking entrances: http://learningstore.uwex.edu/assets/pdfs/G3273.pdf .

Please let us know if you need any further information.