You shouldn't need to do anything. Often times it is the werewolf that has the problem. They need to leave the shrine area, swipe around a bit, then walk back in, usually this allows them to bite you. Sometimes both people need to relog. Unfortunately there is no one single way to fix this, you just have to keep trying. As long as you definitely are not a vampire, and they definitely have the devour and bloodmoon perk, their bite counter is not present, AND you are all standing close enough to the shrine, it should work. Also check you don't have Sanies Lupinus already. Good luck!