Hello Aasif bashir dar. Greetings.
Thanks for sharing your system parameters. But to my understanding this problem should be in no way related to modulation or any such a factor. I think it is the disturbance in your system due which your signal is getting attenuated to this very level and as a result of which you are not getting the required bits. Try using a higher order filter like of the order of 5 or so and check for the results again. I hope that may work.
Regards
Atul Sharma