I had the same issue. found an article that said to cover the flat surface with a rag the tap it with a hammer. It releases the pressure and they go together nice. I was gun shy at first about hitting it with a hammer but was tired of fitting it every time. works great for me.I have a third function installed on my Kioti 3510 using the flat-faced skid steer connectors.
I can never get it connected without completely unscrewing all of the connectors and releasing pressure on both the tractor side and the attachment side.
I never have this problem with my rear remotes (pioneer connectors).
I always release the pressure on the tractor side of the hydraulics...
What am I doing wrong?