i've taken a hiatus from the game, and came back to no riders. i've bought a new fleet of geldings, and now i can't hire any riders for some reason, very specifically LVL 10 riders. no other LVL has given me issues.

it allows me to from the horse, and says "a rider has been hired and assigned," and a rider is never added to the horse, or hired. it's been going on for a day or so now for me.

Horses who reach level ten become schoolmasters and are no longer able to show. Thus they do not need riders and you will not be able to assign one, even though it says you can on the horse's page. However, you can use your schoolmaster to give your riders a training boost when training them, so it's worth it to keep some schoolmasters around.

Level 10 riders are retired riders and can't show so you can't hire any.