If you're using the flashes/camera in TTL mode, then stop doing that and use Manual mode.
TTL (through-the-lens) is a way for the camera to automate the power setting on the flash based on metering. The camera tells the flash to send out a low-power "preflash" pulse of light so it can be metered. And then, based on the meter reading, the flash power output is then adjusted. Vary your distance to the subject; vary the way the subject is framed, vary the background behind the subject, and you could vary the meter reading and the resultant power output setting.
Just as reframing an image in Av or Tv mode can get you different exposure settings, simply shifting the frame with TTL can get you different flash power output. That's why there's a variance. If you want to lock down the flash's power setting you must put the flash into M mode and dial in an explicit power setting that's a ratio of the flash's full-power pulse (1, 1/2, 1/4, 1/8, etc.)
Another possibility, if you are getting a strong pulse on one shot and a weak pulse on the next is that you're simply not waiting for the flash to fully recycle between shots. At full-power, most speedlights can take anywhere from 2 to 7 seconds to fully recharge the capacitor from the batteries. A speedlight is only powered by AA batteries, after all. Most speedlights will indicate if they are recharging and when they're reading to fire again with a light or a beep. Check your owner's manual to see if you're waiting long enough. Using something like burst shooting will typically only work with flash if your flashes are set to very low power (say, 1/8 or below). You may have to compensate with a higher ISO or wider aperture setting to bring the power down to where the recycle time works for you.