>Are you using TTL or TTL-BL? I was using TTL-BL inside and >seeing the same thing you are describing. I switched to TTL >and solved the problem.
This is correct, and a VERY important point.
TTL-BL is for adding fill to an exposure that is primarily from ambient light. If you use the flash in TTL-BL mode, and there is no ambient contribution, the image will usually be dark by one or two stops.
When you want the flash to supply all of the exposure, you should use regular TTL. Then the flash will fire at the proper power to supply all of the exposure.
Also, when using regular TTL, the camera should be in Manual mode with the shutter set high enough to eliminate the ambient contribution and the aperture should be around f/5.6. Don't use a small aperture like f/16, or you may run out of flash power to fully expose the subject.