Dog won't come to leash

We have an 8-month old miniature schnauzer puppy who's doing well with most of the training we're doing with him. He's doing great on his walks- not pulling much at all, and he seems to enjoy being outside for walks. However, recently he has stopped coming to us when we call him to put him on the leash for his walk. When we try to come to him, he starts playing chase with us. I've tried using treats to lure him to me, but that's working less and less. This morning, we had a standoff where I was sitting on one side of the room with a treat in front of me, calling him, and he was just sitting on the other side, staring at me.

In addition to this, we had been training him to ring a bell when he wanted to go outside, but he also now refuses to ring the bell. I don't think he dislikes going outside, because he's fine as soon as we get out the door, so I'm not sure where this is coming from.

Any ideas on how to get back on track? Any help would be greatly appreciated!

drop the lead on the floor and walk away with a loud exhale. YOu just told the dog " I dont have time for this, and this is not a game." Meanwhile put a second lead in your pocket. Go sit on the couch and wait.. When the dog comes back like " what happened mom?- I love this game" pet for a few seconds - attach the lead and say " okay outside ( or whatever word you use..)..
Dogs will make a game out of anything, especially if they are smart... It takes time and patience but this method always worked for me..
*** edit- also - check your body posture when you do the lead on.. Are you tense? Irratated? .. you need to be calm and casual, and relaxed..**

The other option would be something like this- you have a crate- goes in to have cookies, after finished get the lead then go out.. ( your dog is making a game out of " keep away" before..)