Also pay attention that you're pulling the correct dark slide. It's easy to pull the most accessible one on the outside. Just last week I pulled the wrong one, flashing a whole sheet of film that was luckily otherwise unexposed. I noticed it and didn't lose a shot for it but I still felt silly.

If you suspect your shutter/strobe sync, set up the camera and a head or two just looking at a wall. Make it fairly dark without the strobe pop. Get under the dark cloth and look at the groundglass. You should be able to tell if the shutter is open while the strobes pop.