Do both the trip odo. and the total distance odo. not function?
If both are not working, try loosening the cable at the speedo housing about two turns.
If only the trip odo is not functioning, or stops working shortly after being reset, the problem is a shim pack that holds the odo. drive gears in their proper places. I had this problem with two speedos from "Chuckbeckmotorsports". These units were not assembled with any great care. The manufacturer was the problem, not Beck.
I had to dismantle the speedo, shim the gears, and revise my sailor language dictionary.
This is not a job for the average wrench bender. Finesse is required in huge amounts. I would recomend you remove the unit and have it serviced.
Good luck......
Leon C.