At our club we have 12 week "seasons" and Divisions based on 0.25 NTRP increments - and they're Men's and Women's combined.
After each season, the top ranked player for each receives a free ladder session, and moves up to the next division.
Is the scheduling flexible? that might be killing it too. If you have a fixed schedule, they HAVE to show up and manage family obligations, jobs etc around it. If you provide flexibility, people re-schedule ALL the time and eventually the program just fizzles out.