I have done all of those things and more.
- uninstalled 'line', restarted ipad, and reinstalled app
- cleaned screen
- clicked on the manual guideline tool (circle in upper right corner) to turn on and off
- pressed harder on the slide
- turned off ink and unconnected it
- shut app down, reopened
etc.
still it's a 50/50 chance it's going to work. the 50% of the time it doesn't work i get this