Checking the wiki on page
https://wiki.project1999.com/Skill_Tracking it seems:
Druid (Max: 125): Distance = (Tracking Skill) x 10
Druid tracking capped at 50 until level 30, then 5 per level post-30.
So for my druid at level 38, the cap is 90.
He has hit 90 and now it won't move, suggesting the wiki is correct.