This happens typically if the Start cable is bad. Especially if it starts happening after multiple races have been run without a problem.  Replace the cable and the problem should be corrected.  The other possibility is that you are using a starter with a “Normally Closed” start output.  You can determine this by going  into “Test Mode”.  If START is displayed next to the TEST MODE text and disappears when you hit start it means that you care connected to the “Normally Closed” output of the starter.  Move the start cable to the  Normally Open output on the starter.