I've had this issue a couple of times now, most recently when trying to create a ride plan on the Mah Da Hey trail.
If you search for trails on the general trailforks map all of the trails appear as normal, but when you go to the ride planner only certain trails within the Mah Da Hey are able to be added to a ride plan.
Does anyone know why certain trails can't be added to a ride plan, or if there's a workaround?