That is a known bug, the game engine doesn't support attaching a cart to an actor who was already attached, even if the previous cart is no more, (or is the same); you will be detached from cart sometimes in sex scene, or when the haul is finished, to fix it you have to somewhat reset the actor to attach it again, so you have 3 at least options to make it works again in the same session :   use console and use the player.sexchange() 2 times and reattach it then ctlr-e, to detach it
  • 1

    Reputation Points