Make sure the coupler on both cars is set to unlocked. If one of the couples is locked it can cause issues.
When shunting at Greenbriar recently I had a locked coupler cause me some bother as I tried to couple up, just released the coupler the usual way & all should be well.
Yes, this was my problem too. Used Key 8 to access 'free roam' and manually unlocked them. I think it had happened a few times and I so just got used to unlocking first. However, it looks like the lock drops back down anyway before you get to shunt up that last few feet. Weird!
If two cars fail to couple, make sure they are contacting each other, then apply brakes. Use free camera 8 to unlock one of the couplers, then immediately lock it again. You'll see the air hoses join together and the coupling's complete. Works every time.
Press 8 to unlock couplers? So much for the often touted "immersion". I, too, have encountered coupler problems while shunting, of course. My solution is, I think, more realistic: climb down from the cab, walk (or run, if you feel up to it) to the spot of the problem. So far, so good, but now comes the departure from immersion: your avatar doesn't have any arms or hands, so you have to maneuver your mouse or controller button* until you find the right spot to select the correct couplers and then complete the connection. Reverting to the perhaps easier use of the 8 key may be a way out of the difficulty, but I don't see it as a realistic simulation of what a driver without a yard hand available would do. And, yes, when points have to be changed, I prefer to again leave the cab and move the lever "manually" before proceeding. Both of these actions are part of solo shunting; whether that of itself is realistic or not I cannot say, but this and , as far as I know, any simulator requires compromises between reality and the limitations imposed by the simulator. *This doesn't strike me as the correct term, but I use a PC and a mouse.
Seriously? Running 500 to 1000 yards down a consist to check you're hooked to a caboose? That's too much immersion for me!
Each to their own but you have way too much time on your hands. I’ll take the 8 camera everyday. The only time I walk on this is picking up a collectible, there are much nice looking walking simulators available