We have 2 Bromptons, one for me & one for the wife, both S6L's, both less than 12 months old and not had much use, we got them purely for days out where its not practical to take our other bikes so not getting hammered as commuters every day. On mine, the rear mudguard roller spins freely but also wobbles a bit, my wife's is almost impossible to spin. Its nothing to do with the amount of lube, its just a poor design that has also been constructed poorly. Personally I think that's pretty bad quality control for such an expensive product.
I sourced a replacement with a proper bearing from Bikegang for about £5 per bike which arrived yesterday (came much quicker then I was expecting too) and they are MUCH better than the part fitted (I don't know why they cant be as good as this from the factory). I'm yet to fit them but they spin very freely and are a direct replacement for the existing part. Worth a look if the existing part is giving you problems.