Snap to trail is crazy erratic, am I doing something wrong?

I'm kind of evaluating CalTopo, main use case being planning and evaluating routes in unfamiliar areas. So I want to create a route then look at the distance and elevation profile.

I'm finding that snapping to the trail is ridiculously erratic, like sometimes it refuses to snap on a route that I'm hovering over, or it will refuse to select a segment of a trail and just starts a new segment, or it won't continue from previous click to where I'm hovering the mouse but instead selects a ridiculously roundabout route. So I have to do a bunch of really short distance clicks, sometimes drawing a straight line to approximate the trail that it won't select, sometimes taking the trail a tiny bite at a time so that it will snap to it. It's not really working for me. Am I doing something wrong? I'm typically on the Topo or Hybrid base layers.

Also, as long as I'm asking for help ... if I save a line by making only two clicks, one at beginning and one at end, and then decide that I don't like it, can I drag the endpoint to a different location and have the line re-snap? It seems like the answer is "no", that snapping only works during line selection, but the line isn't saved as where you click but actually as all of the waypoints along the way ... is this correct? If so, is there a way to erase a portion of a line?