I'm sure this is a simple thing but it's driving me crazy. Every time I draw a line it gets erased as soon as I try to draw the next one. Any ideas?
If you click 3 times with the Line Segment tool (moving your mouse between clicks) you get 2 unconnected line segments.
If you do the same with the pen tool you get 2 connected line segments.
Whether the segments are visible or not depends on whether they are coloured or not.
Test this by turning off the preview (Cmd/Ctrl+Y). Your ‘erased’ lines will probably show up.