Since you didn't share any code, I can only guess what's wrong.
I suspect that you are setting both trackSkin and secondaryTrackSkin. In this case, they are displayed at the same time. The trackSkin appears on the left side of the thumbSkin, and the secondaryTrackSkin appears on the right side of the thumbSkin. They meet in the middle of the thumbSkin.
On the other hand, the default theme sets only trackSkin, but not secondaryTrackSkin. When there is only a trackSkin (and secondaryTrackSkin is null), the trackSkin spans the entire width of the ToggleSwitch.