Our new Lidls are always trying to push people to use the self service tills. They are ok if you have half a dozen things but a trolley full is a nightmare when stuff doesn't scan etc. Sometimes there are no ordinary tills open, but when they are there is usually a long queue. Eventually they open another but then close it after 10 minutes or so or like you say switch to a different one. We have switched back to our old store as they only have normal tills so you actually get served. If I'm spending my money in a shop the least they can do is have enough staff to serve people.
Our Asda store has just been refitted and they have done away with at least half the ordinary tills, added more self serve and a new self scan system where you scan goods with a handheld device as you go around and pay at the end. How that's not open to shop lifting I don't know.