It is certainly very discouraging for a racer when their car can’t even make it to the finish line.  It can also cause you some slow downs in your race, as you deal with the DNF cars.

There are actually some easy things that your race crew can do to help all of your cars finish:

Check-in:

1. Visually inspect for lube.  This is easy if you have a "graphite only" lube rule, as you can see the graphite on the car.  If your rules allow for liquid lubes, then you may need to ask the racers (better yet their parent) if the car had been lubed, if you cannot see signs of liquid around the wheel bores.
2. If a car is below 4.5 ounces make the racer add weight.  Make sure that you make available a glue gun and some extra weights (washers, BB’s, fishing weights, etc.) in your pit area. 
   Note: Some weights that are great for quick weighting are wheel weights (for real cars).  They come in 1/4 ounce increments, so it is easy to determine how many segments you need.  Then peel off the tape backer and stick under the car.  These can be found at a Harbor Freight store or online.  You also might be able to get some from a local tire shop.
3. Make it a part of your check-in procedure to test each car down the track (individually and not timed).  Any car not finishing should go to the Pit Area for a "tune up".  If it crosses the finish line, then and only then, it is officially checked in.

Racing:

• If a car doesn't finish a heat, consider sending it to the Pit Area for a "tune up" before its next heat.

For the races that we run, we do these things and we rarely have cars not finishing, unless they have a serious wheel or axle issue or a wheel falls off.