Help a numpty

I have only recently ever used a proper flash and picked up one of the Sigma EF-610 DG ST when SRS were doing a deal a couple of weeks back. My problem... When it is mounted on the K-7 with the Sigma 70-200 attached and the camera is set to continuous shooting the flash fires with every shot, but when I mount my DA* 16-50 it flashes every now and then, certainly not for every shot. Any ideas?

maybe it's the siggies talking to each other

Could be to do with the power output required for different focal lengths. If you are shooting wide you need more power as it has less range. This increases recycle times and you only get one flash every 3-5 seconds rather than 3-5 per second.

Thanks Dan, that seems to make sense.

If Dan is right, you should not see much difference between 50mm and 70mm.