I did this with mine, it worked on the second day I believe. but I left them for an extra day to make it stick! Maybe try the flashlight trick? After lights out , stand a small flashlight ontop of the glass top ( if you have one, if not go MacGyver on it.. ) they are said to wanna be in the beam of light .. worth a shot
Do you have other clowns hosting? I had a pair that couldn't care less.. until they saw another pair hoarding their nem.. then it was game ON!.. even with 7 nems in tank they had to fight it out for that ONE unlucky Nem.. lol