Can't to get measure to accept any hits, always getting red arrows.

Check cable connections.
Make sure there is not too much wax
First try to use the re-practice button and practice again then it will come back to measure automatically and try to make measurements again.
If you are consistently getting double hits, try tapping harder both in measure and practice. If necessary re-practice.
If unable to get it to stop go back to set up and the sensors tab and uncheck all the trap boxes at the bottom of the window.