OP, you have a right to be frustrated because there are various reasons these couplers dont work so well. Unfortunately, some of us that have overcome the reason our particular setup didnt work for us seem to think that we have discovered the universal solution for everybody elses setup as well.
Case in point: I tried all the conventional advice here:
A) Equalizing the attachment's internal pressure by connecting its couplers together until the next time.
B) Relieving the tractor's internal pressure by whatever the manual says, usually with the engine off but the ignition partially on.
Neither one of these solved my problem on the flat faced couplers on EITHER my Kubota or my Gehl tracked loader. They let me get the couplers a little closer to connecting but not quite all the way.
What did solve my problem on BOTH of these completely different machines is holding the corresponding valve open while connecting the previously stubborn couplers. (Of course this is with the engine off and the ignition on)
If I knew how to post a video and if I still had a wife, I would video my struggle to make the connection and then how they slide together like butter once someone comes out and pushes the valve open button for me.
My theory is that even though there is no residual pressure in the tractor's or the attachment's circuit, there is a trapped volume of fluid between the coupler and its control valve that will not compress when the coupler needs it to. Holding the valve open will give that trapped fluid a place to go.
It could also be that on new tractors and on connectors that are rarely used, the connectors are too tight to allow any necessary leakage. This might be why people with well used equipment dont have this problem.
Holding that valve open requires a second person but I keep a c-clamp handy if I am by myself.
Also, I bought some couplers advertised as self-relieving from Discount Hydraulic. They werent too much more that a pair of regular flat faced couplers and they worked every time so far on the Gehl. I havent installed them on the Kubota yet.