Maybe it's your timezone. There are times when the public jam sessions are empty. I just join an empty jam and play by myself sometimes, that way other people see me and join.
The best way to find people to jam with is by joining the weekly jam session on Saturdays at 19:00 UTC:Weekly jam session on May 17th
Or you can announce your own jam session on the “Jam Sessions” sub-forum:http://forum.jammr.net/6/
If you announce a day or two in advance that should give people time to read about it.
Thanks for using jammr,