wgiles hit upon your most likely culprit, which is sync delay.Shutter speed has no effect on your meter reading when using strobes in a darkened environment. From an exposure standpoint, f/8 @ 1/4 power is equivalent to f/16 at full power. In general, when using studio strobes it is a good rule of thumb to use a shutter speed that is at least one full step below your camera's maximum sync speed. In the case of your 20D (max sync speed of 1/250th.) that would be 1/125th. of a second.
You didn't describe what was happening when you used a shutter speed of 1/125th. but there are three possible outcomes:
1) Everything looked fine
2) Only part of the frame was exposed correctly. The remainder was black.
3) The exposure fell off noticably from top to bottom or bottom to top (depending on which way your shutter travels). That is, uneven exposure from top to bottom or bottom to top.
Since you posted, I'm assuming you didn't encounter case #1. If you experienced case #2, then there is too much delay in the triggering chain. The strobes aren't receiving the signal to fire until after your second shutter curtain has started moving. Selecting a slower shutter speed will solve your problem. If you experienced case #3 then the strobes are taking longer to discharge their energy than your shutter is staying open. Your shutter is closing before your strobes have a chance to release all of their photons. The result is that there's a graduated area of underexposure as the second shutter curtain travels across the sensor plane, depriving the sensor of light. Selecting a slower shutter speed will also solve this problem. So will reducing the power of your strobes, so that they need less time to discharge their stored energy.
If there was any documentation that came with your remotes, see if it says anything about sync delay or recommends a maximum shutter speed. I'd be suprised if you needed a shutter speed slower than 1/60th.
People who do stupid things with dangerous substances often die! -me