I’ve now tried the first two and they worked fine as you advised.
The third was due to the scenario you described so I’ll try to keep sketches simpler.
the fourth was related to not being able to unlock some points that overlap on a series of tangent circles that I’d locked and then tried to undo. Basically, a total beginner trying to grasp how to do things in 3D CAD. I have a long way to go!