I am guessing that you are shooting inside in low light right? Sigma lenses are just fine. Your problem is your shutter speed. In low light you will have to open your lens up as far as you can or use a powerful flash gun. I would say for inside sports you will need an FStop of at least 1.4 to 1.8 to get a good sports shoot inside. I am guessing your package lenses are 5.6 maybe?

Try upping the iso a bit,say to 800 or 1600. To stop most action, you will need a shutter speed of 1/250 sec., so the aperture must be fairly wide even at iso 800.

I think that a basic photography course would be a valuable investment for anyone who wants to advance past snapshooter status. All the auto program settings on newer cameras can't substitute for a basic understanding of f stops, shutter speeds, ASA settings, depth of focus, etc. A lot of my 35mm slides go strait from the light box into the trash but 99.9% of the time I know where I, not my equipment, went wrong.

I have been using a Sigma 24-70 f2.8, Sigma 70-200 f2.8, Canon Speedlite 550, and a Rebel to shoot high school sports. I shoot in Tv and set the camera for 1/100-1/200, ISO 800 for all sports. So far, this combination has worked well for me, stopping the action, sharp.If the sun is still up I drop my ISO lower.

Could be a few things going on but anyway..... is that lens a "fast" lens?
Depends where you are shotin' it(inside/outside-light available) but you need a good fast lens for some of those shots
Try additionally to get closer to the action thereby not having to utilize the lens at its maximum focal point and "slower" possible lens specs.
Someone said about moving up the ISO speed to I think.
And shutter speed .....move it up.
Additionally if on predictive AF try switching to manual focus(good experience)
Sometimes predictive not as good.