Are you using the same 'form' variable in user step that you used in first step? I would suggest that you put the Variable Logger as the first step in your render process to check if it gets called several times or not.
I'm writing to the log file every time that my custom service is invoked.
When I open a form for a first time in workspace I see that it was invoked but when it reaches "assign task" activity and opened again the service is not invoked.
I have only one form variable.
I forget the exact name, but I believe there is a "render only once" option when you configure the render service.
Before the assign task step the form was sent as an input to another process and then returned as an output.
In this process the form was assigned to another one that was not configured to use custom render service.
Thanks a lot!