Might it be the collie breed too perhaps? We had a border collie who had the recall of a gerbil. No I stand corrected, the gerbil would have been easier. Some of teaching a dog to come is voice tone too. I hear people in the dog park all the time begging their dogs to come to them "Come on Sheba, let's go COME SHEBA somebody go get her SHEBA COME we have to leave Sheba come now Where did she go?". Ideally you want them to learn you call their name in the right voice and that's it, they're right beside you instantly.

Lots of good advice already.

I've always called "bye, bye" in a very high pitched voice & walk away to all my dogs & it's always worked. Never knew why as there was never any training with this command.

The other thing I did when Toffee was young, Walk in opposite direction & call "follow me" & drop a few treats on the ground as you go.

I also at random turn & walk in opposite direction, this teaches a dog to keep a eye on you. Good luck.