It sounds like the outside edge of the groove has been rounded off by the string repeatedly running over it.
Get some round needle files from a hardware store and carefully deepen the groove.
When you are satisfied with the results, lubricate the groove with pencil lead.
Only take a little off at a time so you don't ruin the spacing.
It's a good idea to practice on some scrap wood to get a feel for how much pressure you'll need to use.
The needle files will come in useful later if you need to adjust the nut on your or a student's instrument.
Oh yeah, when you move a string in and out of a groove, be sure to grip it on both sides of the bridge and lift it out by applying downward pressure on the bridge with your thumbs so it doesn't touch the side of the groove but gets seated directly in and out of the groove. (Hope that makes sense.)