Why so many games cant be added in multiples?

Im trying to back several football games in a multiple coupon. Im choosing one team to win and then in the other game another team to win. Only one back per game, not lay. On some games it accepts but at least on half it says "Some of your selections cannot be combined". It shows the "cant be combined", even if I only choose one team, before I add another one, so some games just cant be added.

I saw this in baseball too. Im only trying to back and only one team per game. Some it accepts some it doesn't. I saw that some baseball games where there was only a 200 euro market, it wouldn't accept but if market was 1000 euro and more then it accepted. But, i'm trying to add multiples in a football that has 20000 euros already matched and even that it says cant be added, even if i have nothing added and this is the first one.

Why is it like this? Thanks.