Strobe output issues

Hi...a client of mine has a Canon G12 with and Ikelite Housing and dual D125 Strobes. The strobes seem to function normally at 1/8 and 1/4 power...but as soon as they are turned up to 1/2 or full power the shots come out almost black. This occurs whether on manual or automatic settings. I spent hours trying to trouble shoot this yesterday with no luck and am starting to think it might be the circuit in the hot shoe. The strobes and sync cord function perfectly when attached to his Sony compact camera. Any thoughts or ideas...something I am missing? Thanks for any help!

I don't know the camera well, but perhaps it is pre-flashing. And the strobes are able to recycle something fast enough at the lower power, but not at half or above. Does the strobe appear to be flashing at the higher powers but the light can't be seen in the image? This would suggest the strobe is being triggered by the pre-flash of the camera and cannot recycle fast enough for the main flash.

Thanks for the reply! I did check if it was set to pre-flash...and it wasn't. So the problem persists unfortunately. Seems that the whole set up needs to go in for service.

Have u tried changing the shutter speed?