How to get cheese sauce to stick to meatballs
A very hot sauce that is thin and runny will thicken as it cools down.  That's why one makes a sauce thinner in the pot than one wants it on the plate.  A solution to your problem might be as simple as serving the sauce at a lower temperature.  If you're serving the sauce on a heat source, lower it.

If you can't alter the serving temperature for some reason, an alternative is to increase the thickening agent, whatever it is.  Increasing something like flour (starting with more roux) or cornstarch wouldn't affect the flavor. Or possibly a different thickener would be better for your recipe. Arrowroot might be a better alternative if it's an acidic environment, for example.  There are lots of choices.   

A thicker cheese sauce is easier to scorch, though, so that's something to look out for.