I don't do it often but if I've started a sketch, then decide I want the lines and arcs mirrored, but not copied about a line, all the relationships and definitions are usually lost.
I then go through the sketch adding back in all the horizontal, vertical, tangential, equal, dimensions, etc...
I have used the Tools - Dimensions - Fully Define Sketch button with some success.
Depending on the complexity of the sketch this all can take some time.
Does anyone know if there is a setting I ought to change somewhere, or is this how it is?