Ah the sit stay, hate it seriously I do.
I would go back to square one. Put him in the sit and pivot out in front of him, slight tension on the lead, treat and pivot back, then 2 steps , then 3 until you across the ring. Then do the same thing without the long line starting back at one step, 2 step. Then go to moving about the room but don't leave, go back and reward every 10 secs or so. then start increasing the length of time between going back. When you can do the entire 3 mins with you wandering around the room, start going out of sight. Go back to returning and rewarding frequently and then increase the time you are gone.
If he gets up ,say nothing, just take him back to his spot ,set him up again and leave. Also make sure you are not unintentional teaching him to break sits or downs at home. If you tell him to sit always make sure you release him. IE if you are getting into the car with him and you tell him to sit ,don't let him stand up on his own, release him with ok or whatever your release word is or another command. Sit or down should always imply the stay unless you release them. Hope this makes sense.
Practice sits and down randomly around the house on the couch ,on his bed, on tile, on dirt ,on carpet , on the front seat of the car, as the windshield wipers are going , praise and treat often build it up over time and if he breaks just replace him to the position he should be.
Hope this helps. Justice went though a period where he would break his sit stay everytime for 6 months. He would lay down. It was stress and he thought he could move. I found out the first day I paid attention he was getting up out of sit and downs at home very often, once I starting paying attention and releasing him instead it got better right away , the rest was practice practice practice and don't stress them out(no corrections just put him back).
My mom was visiting last month and when I was at work she told Justice to sit when she was doing some cleaning and she came back 30 mins later and found him still sitting in the middle of the living room where she left him. She thought weird why his is still sitting there. Opps! She remembered and quickly released him.
It can be done just keep at it.
