Maybe there are two types of contragrav generator, a cheap, relatively lightweight one and an expensive, relatively massive one.

Cheap (landspeeder-scale) generators depend upon taking in natural gravitons, then rotating their vectors 180 degrees and using them to push against a nearby surface. Above a certain [very low] vertical threshold, these natural gravitons aren't strong enough to supply this effect.

Expensive (ship-scale) generators manufacture artificial gravitons and thus don't have a service ceiling (or have one that's so high as to be irrelevant, since reaction thrusters can take over from there).

NOTE: Since the gravitons from both types of generator can push against a nearby surface, both can provide reliable if inefficient thrust when within a few feet of a planet's surface (e.g. achieve automobile-like rather than airplane-like speeds).