Good morning, Hoffy;
To promote longer life with your electronic flash units, use them. If you are not using them, pull them out of the bag once per month, put them on an AC house mains voltage power supply to conserve your batteries, and let the thing charge up for about 20 to 30 minutes. Then fire it and let it charge again for another 20 to 30 minutes. Repeat this about three times, then turn it off after the last firing and put it back in the bag. Repeat with the others in the bag.
This will help to keep the main power capacitor for the flash tube properly "formed" so that it can safely and properly hold a full charge for the flash tube to use when it is fired. If you seem to be getting a weak light output when using the recommended camera settings for the flash unit, and the camera seems to work properly with another electronic flash unit, this could be the problem. You can try "reforming" the capacitor using a longer process like the maintenance process described above. If it does not come back, the only real cure is to replace the capacitor, or the entire flash unit. Yes, there is routine maintenance even for electronic flash units. We did not need to do things like this back when we had flash bulbs.