Does the topo map work on your computer? Have you compared it to what you see on the GPS? In other words, are you sure that you're really looking at the topo on the GPS and not the basemap or some other map? You are aware that when you send a map to the GPS from Mapsource, it erases any map that you previously sent - right?
Otherwise, a couple other ideas come to mind. First, all the newer models have a bug with the profiles feature that can prevent elevation contours from showing. On the GPS, check to see what profile is active. If not Recreational, switch it to recreational. If it's already recreational, switch to something else, then back to Recreational.
Finally, there is at least one map - maybe more - on this site where the countour lines are contained in a separate transparent map that must also be installed. What map are you using? And have you tried more than one topo? Do they all have this same problem?