Used sketch, one minute, & now it wont draw

I've just learned how to simply use Preview sketch tool to draw a line. My Mac Mini (ElCapitan 10.11) has Preview 8.1. It worked fine for drawing a line on a road map, to indicate the directions for driving somewhere. I also succeeded in changing the line's color, and thickness. But many times when I used the pen-point cursor image, it would start to drag a rectangular box. Okay, that box did have a line going corner to corner, but by clicking the sketch tool command a few times, I could get lucky and return to the free line-drawing usage. Minutes later though, when I tried to draw another line, I either got only a dot not a line; or dragging (still a pen-point), I could only select part of the image, in a box, that box might then turn color or be deleted if I hit delete.

I don't know why i cannot draw any more lines. My searches provided skeletal instructions for first time use, or for other versions of Preview. Preview 10 seems more like real drawing, Preview 7 has a cruder drop down menu.

Nowhere could I find any reports of the pen-point (sketch) cursor making dots only.