Modern DSLRs and their flashes use a monitor pre-flash to establish how much flash they need to use. This preflash is virtually indistinguishable from the main flash. This preflash fires the 10K early and its light is gone by the time the mirror flips up and the shutter opens.
Since the 10K fired with the preflash, the camera thinks there is a boatload of light (not knowing about the 10K) and adjusts the speedlight accordingly. When the actual exposure flash is fired, there is no 10K flash to help, and you get an under exposed image.
To work around this, simply put the speedlight in Manual mode. You can also reduce the power in this mode to help preserve batteries, and increase recycle time.
Should your speedlight not allow you to set it to manual, you can use the * button and set flash exposure lock. This fires only one preflash, and uses that value from shot to shot, firing only one flash each time. You may need to reference the camera's manual for specifics on how to set this up. I would also suggest using negative exposure compensation for the flash as well, again to reduce battery consumption and speed up recycle times.
Ultimately, I would suggest getting a radio remote system like our CyberSync (CST and CSRB).
http://www.paulcbuff.com/cybersync.php as it solves several problems that can arise when using a cable or slave eye.
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