My guess would be that it's an issue of recycle time, these flashes will fire a second time even if not fully recycled but in this situation they will fire at a reduced power... This would give the results you're seeing. Have a look and see what the recycle time is for the flash and then try and work with that, be warned the recycle time will get longer as the batteries lose charge, fresh batteries will allow the flash to work at it's best, so a few sets of good rechargeables will be a good investment.
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