I have never seen a sim decline an ex game.

I'd send in a ticket...they'll either tell you user error or fix the bug (I think it must be 1 of those 2 things).  This is definitely nothing I've experienced in my few years of coaching.

Have you considered that you're not trying to schedule the exhibitions until after the first exhibition has already played?  The first exhibition game is the 2 PM immediately following the end of recruting, so you only have a few hours after recruiting finishes to schedule it if you haven't done so during recruiting.  Given what you said happens and the fact that your 1 exhibition in Knight was on the 2nd day AND you started complaining about it on the forums 5 hours AFTER the first exhibition games had been simmed, I'm guessing that's your problem.

I'll bet that's it, dahsdebater.

Also there's a deadline. I think you have to schedule at least an hour before the game is to played.

That could be the problem. Maybe in future I'll try to schedule sooner and it will fix it.

Thank you for pointing out what could be my own (foolish) error. We'll see in the future if it stays like this or that fixes it.

If you're worried about team quality of who you play and that's why you've scheduled later, know that you are able to schedule Division 1 and 2 as well.  Even a crappy sim D1 team will usually be better than most D 3 squads.