With an Iron fireman stoker, which is similar to the Prills, there is a small return pressure line back to the hopper to keep it supplied with a positive pressure. The lid has a rubber seal around the loading door for the same reason.
Is the smoke leaking out of the hopper when the stoker is running, or when it is idling, or both?
If when idling and there is no Prill combustion fan running, then I'd say you definitely have a draft problem. If the smoke comes out when the combustion fan' is running, then maybe the auger is getting worn, and the comustion gasses are backing up the feed auger.
Beg, borrow, buy or steal
a manometer, and use it to monitor your draft, so you can watch what the draft is when the hopper is leaking smoke.