Consider:
1) clean wheels of debris, graphite, and wax with mild abrasive pad/micromesh
2) check rear alignment
3) check steer; add more
4) rear wheel gaps too large; tighten gaps; check front as well
5) move weight forward (only adding a few grams to the FDW)
6) check rear wheels; hubs or edges deformed; replace
7) bore diameter not equal when measured at inside and outside hub; replace
8) re-polish a new set of rear axles (diameter is too small)
9) re-polish a new set of rear axles (rear axles slightly bent)
Could be any one or all of these things. 1 thru 5 are easy to do and usually can correct most problems.