If that is the case, obviously your previous employer did not comply with the rules concerning the Maternity benefit of its employees. As in your case, malamang employed ka pa nung nag-notify sa company na manganganak and since your company did the part ang kulang talaga is walang advance payment na nangyari. Pde ka na ngayon mag-file ng complaint sa SSS para isuplong ung previous employer.
The L501 you are referring is one the requirement of the SSS granting its employees in claiming benefits such as maternity, sickness etc.
Hope nakatulong ito sa iyo. Also, just a reminder malaking halaga ung hindi binigay sau and may you have the guts to pursue your claim.