What is the Grid size of the baseboards you are using?
A small brush size (up to about 5m) on a 10m Grid baseboard will only work at the
intersections of the yellow (10m) grid lines and NOT anywhere else. Brush sizes from 5m to under 10m will work best on the yellow grid lines and slowly/poorly anywhere else. Brushes of 10m and above will have no problems. This has always been the case with Trainz.
A very small brush size (up to about 2.5m) on a 5m Grid baseboard will only work at the
intersections of the grid lines (both yellow and grey) and NOT anywhere else. Brush sizes from 2.5m to under 5m will work best on the grid lines and slowly/poorly anywhere else. Brushes of 5m and above will have no problems.
In Trainz Plus using HD grid the smallest brush size you can use is 0.12m (12cm) and it is not restricted to grid lines or intersections.
See
Ground Brush Resolution for more details.