So the bearing is not rolling?
If this is say...a crower rocker arm, maybe the customers are using improper assembly lube which may cause the roller to not roll during that important first few minutes of operation.
Since oil is super slippery, it doesn't take much resistance to rolling to stop the rolling action without causing any damage to the needles...so the outside of the bearing glides on the oil film.
Once there is even a slight flat spot on the outside of the roller where it contacts the cam, it may stop rolling and now acts as a "pad type cam follower" instead of a bearing because oil is such a good lubricant that it allows the cam to turn against a stopped roller causing a flat spot to get worn eventually instead of causing damage that snowballs into galling and or catastrophic heating.
If a hydraulic "lifter" is employed, the bearing could stop turning every time there is oil pressure to keep it against the cam with any amount of pressure while a mechanical follower may allow turning when it's not under pressure of opening the valve.
In stock factory usage, you have less chance of the wrong lubricant being used but in the aftermarket horsepower world, every guy has his own recipe and many are just wrong.
If a customer can send back the rocker arm and the camshaft, you could install it in an engine and just watch the roller to see if it's stopping and you should easily hear the "thumping" if it is.
Maybe a customer can make the observation himself and let you know what he's seeing.
