I also agree that if the wheel cleaning did not fix the problem, the next step will be to send it to BLI.
Just for reference I'll show what I use for cleaning wheels.
This is the Minitrix (now Marklin) wheel cleaner. It is great for removing stubborn gunk on the treads. Brass is a fairly soft metal, so it will not scratch the tread, but it could scratch the wheel blackening layer (which normally gets worn off in use). The nice thing about it is that it also powers the model, so the wheels turn while being cleaned. All you have to to is hold the model over the brush. The bristles do not seem to damage traction tires. For non-powerered rolling stock I just lightly roll the wheels over the brush.
This is the setup I copied from a friend of mine. I use it for gentler wet cleaning. I use Bounty paper towel squares because it is a 2-ply strong paper towel. I moisten it with 99% Isopropyl alcohol. I then power the track and hold the model so some of the wheels are off the paper towel so the model gets power and spins the wheels over the paper towel. For non-powered models I just roll them back and forth several times over the moist towel.
