As the get hotter the element reistance of the glow plug increases. If you are getting say 1 ohm readings on some plugs and at the same time basically 4.5 ohms or so on others.

It is a pretty sure thing you have to examine things more carefully. Since there may be variables present. Pulling them and doing a direct voltage test application for observation may be best.

If a plug and its feed wire from the timer box is showing 4.5 ohms something is not normal.