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Are you using canon camera? I have used a variety of canon cameras with nearly all their flash units over many years and almost all combinations can give under-exposure, sometimes to an extreme degree. It is a 'known issue' and many people have tried over the years to get canon to sort it out but with limited success. the last time i sent a camera/flash combination to the canon repair people they said everthing was working within normal parameters "but you may want to try using exposure compensation", i.e. they knew full well that their system sometimes does not work properly.
Having said all of this, the current canon setup I have (400d+mt24+sigma180mm) is working more or less ok most of the time, it does still have its off days where it will require up to +2 stops for no apparent reason but can live with this on digital so long as the insect has not vanished!