There is, most likely, nothing wrong with your disk.
All of our diamond disks come with a magnetic backing and, as magnets are want to do, they will attract metal objects.
Oftentimes, the scratches you are seeing are coming from foreign objects embedded in the back of the magnet. Even small objects that create a high spot on the top of the disk will cause problems. The first step is to be sure that the back side of your disk is clean and free of debris. If you don't maintain your disk, then you might have a buildup of rust, dirt, debris and possibly glass and metal bits embedded in the magnet. This is no good.
Clean off the magnet with Alcohol, turpentine, or acetone. You might find that you will need to use a Scotch-brite pad or some other abrasive pad to really get the built up debris off the magnet. Once the magnet is clean, go around and dig out any foreign matter out of the magnet with a knife blade or X-acto knife. Once this is done, check your machine wheel head to make sure it is free from debris and rust buildup and try again.
This should solve many of the scratching problems you may be facing. If your disk is clean and you're certain that you don't have any objects embedded in the magnet, there are a few more things to try.
The first culprit we most often run up against is too much water use. Your disk isn't on fire, so stop trying to put it out with too much water. All you end up doing is hydroplaning on top of a layer of water. It might seem like you're grinding along nicely, but you're actually floating on water and every once in a while, you will displace enough water to "grab" a diamond on the disk and give yourself a nice deep scratch.
Dial down the water a bit and try again.
A good rule of thumb is to lower the water down to a point where you notice a slight buildup of white ground glass around the edge of the disk. Then bump the water up just enough so that buildup disappears. This is a good amount of water for the disk you are using. Remember that as you use a diamond disk and it wears down, it will require less and less water, so your water usage is a constantly changing variable and keeping on top if it is vitally important to efficient grinding.
The last culprit for scratching disks is a bit rarer and usually easy to diagnose and repair. If your magnet side is clean of debris and you are still experiencing a stray scratch on your pieces, but it appears to always happen in the same area of the disk, you may have a larger diamond that stubbornly refuses to crack off with it's fellow brethren.
Most of the time, our diamond disks wear pretty evenly and the diamonds crack uniformly enough that the disk grinds consistently. Every once in a while you'll have a diamond that does not crack off with the rest and stays a bit higher than the surrounding diamonds. When your glass goes across that diamond it will leave a deeper scratch in your piece.
All you have to do is locate where on the disk the scratch is happening, then turn your machine off and look around the disk in the area where the scratch appears. Somewhere along the circumference of the disk you will find what looks like a comet tail of glass coming out from behind a single point. That is most likely your offending diamond. You can use the point of knife to "pop" the diamond out of the surround nickel or the edge of a hammer head to manually crack the diamond off. Both methods work pretty well and will get your disk running evenly again.