Verify correct cable. The wire may be too short even if the sheath is the correct length and has the correct ferrules on both ends.
Hook the cable to the wheel, disconnect from the speedo, spin the wheel, and make sure it is turning. If so, dosconnect from wheel and hook to speedo, and give the wheel end a hard twist between you fingers to see if speedo works. If no, wouldn't hurt to try to take the speedo apart and figure it out. Speedos hardly ever break.