Creating a train ( how to follow driver )

I have problems creating a train of drivers, before sprint. I see option follow driver but it is unavailable ! Sometimes i can follow my drivers, sometimes i cant, dont know why, following if someone is attacking is working perfectly

As u see this Godd... guy needs to follow Casper and when i select Godd... and right click on Casper follow is gray , cant be selected.

Plz help

You can only form trains failing 18 kms on flat stages, in the other stages (hills and mountain) will not be failing 18 kms. You can also make a train before the intermediate sprint.