If you switch from both to "L" or "R" and get no drop it normally means that you have a broken wire on one mag so it is permenantly live, usually here you will get a drop on one side as that mag earths and is switched off but no drop on the other side because it does not - it is possible to get no drop either side but this would be
very unusual. The thing you are more likely to see is a larger than normal drop, this is usually caused by a plug not working, either through oiling up or another problem. Any of these problems should be resolved before getting airborne.
