Flex Queue Bug or it's normal?

As I played my Solo/Duo's i got ranked in Silver II my firends are Gold III and Gold IV. So I played Flex with the Gold III we played 4 games but now I cant play with other Flex just with the Gold III. We wanted to invite the Gold IV but it says "Unable to invite "name" your ranks have a to high diffrence" or something like this. Now I wanna know why I cant play with someone else.

For the people who forgot that we're just ranked in Solo/duo not Flex Queue