I don't know if I understand you correct. 

Game shows (competitions) runs every 6h. When this changed I have no idea because I had my account on freeze from spring until 4 weeks ago. Since I came back I have noticed that player shows show up on your horses show page first when it's 6 left until they run= the same time as the shown game shows. If you use the shows arena page you can find all player shows created.

I think i get... A few horses of mine don't have a "show tab", it's empty, only leaving halter shows. Maybe all the shows of that day is already runned then? 

Yeah if there are no shows that means they are currently running. They run I *think* 7 am, 1 pm, 7 pm, and 1 am

A few things to check -

1. The horses have a 'DISCIPLINE' set.  (Flat Racing, Cutting, Reining, Dressage, etc etc.)

2. The horses 'DISCIPLINE' checkbox under 'Working Paths' on their edit page is selected.

--- Both of the above will have discipline shows actually showing on the horse.

3. If there are still no shows, can you find shows manually by going to Town > Shows Arena and finding a show?  Are there still none?

If not then I think the answer is that the shows for that particular discipline/tier are full.  While halters shows will run and put another halter show up, I don't believe discipline shows are the same. Once they are filled, that's it and they go away until the next show window.  But they also hold a lot more horses per show. (I'm not 100% positive on this).

You can ask in the discord for shows of a certain discipline/tier to be created by other players if there are none. 

Then It is! Thank you guys!