They lost their growth factor. It doesn't take very much neglect for this to happen. All it takes is for them to stay in the same solution for a day or two longer than they can stand and then boom, they are pickled and you're done. They may continue to make water kefir but don't grow.
Another thing is that just like kombucha, the scoby grows the most toward the end of the cycle. So you will not see growth if they are not completing their cycle. That said, you cannot restore growth if the growth factor is already fried as you found out.
This is supposed to be a one time purchase but I replace them all the time. People go on vacation and want to know how to store them. Well that's like storing a dog. Don't feed the dog while he is in the refrigerator for a week and guess what? No, you take the dog with you or find a sitter. People aren't going to do that with something like kefir grains. It is easier for them to reorder.
I just thought of two other reasons water grains can lose their growth factor. One is in your control and the other one out of your control. Soap residue. Not everyone uses Dawn dish soap. You are asking for trouble if you change what you use to wash the dishes. Dawn is safe. The other reason could be cross contamination with other brews you have going in the same area. Yeast can coat the scoby and you are done. Different yeasts abound from kitchen to kitchen whether you are brewing or not. I'm guessing it was not your fault. But there is definitely a reason. I only try to trouble shoot putting the customer through the inquisition when he wants to try it again. This is supposed to be a one time purchase.