ash hopper

I am working with a boiler (coal fired, Chain grate stoker type). We are disposing the ash in clinker form , this clinker is stored in hopper ,then after some duration this is released from hopper.

Problem is that during this duration clinker gets sticked in hopper and we have to hammer on sides of hopper to remove sticked ash clincker, is there any solution for this problem .

please give me solution.

thanks