It's hard to judge why the low turn out for the last couple months. The heat and rain all summer could have played a big role. A fall/winter series could do much better.
Something that could help is posting the scheduled races with more lead time. Myself and others prepare our race schedules far in advance. It can be difficult making a race if there is only a week or so notice.
Another idea is if you want more people at every race then have less races. The cost of maintaining the machines, gas, tires, etc to race every 2 weeks can be difficult on the wallet.