I don't think ISO is relevant here, if the OP is shooting in shutter priority the image will simply underexposure if the ISO is maxed out and the lens is wide open.
A few possible causes come to mind:
1) Not shooting is AI Servo. When focussing on a static object the camera auto focuses and then takes the shot, this is fine. When focussing on a moving target (especially a fast one) the delay between the focus being attained and the shot being taken means that the target is now out of focus. By using AI Servo it forces the camera to continually focus on the subject, thus when the shutter is pressed the target will be on focus.
2) The lens can't focus quick enough to keep up with your subject. This is quite possible but you would at least get the odd keeper.
3) The panning movement is not smooth. Apparently one of the most common issues with panning is that the shooter stops moving as they press the shutter, you need to continue the panning movement until you've taken the image. A bit like a golfer follows through the swing even though they already hit the ball.
Things I've found that help:
Turn IS off on your lens. Higher spec lenses have 2 mode IS, but your lens doesn't (if it actually has IS at all). The IS can interfere with what you are trying to do.
Support the lens from underneath with the left hand, holding the camera with the other. Hold your left upper arm to your body to give some stability.
Use back button focussing. By changing a setting on your camera you can use the AF-ON button to control the autofocus system, this is primarily used to separate metering from autofocus but I found it helps, maybe its because my grip changes on the camera and gives it more support.