I've noticed that arrows which used to lightly snap onto my string will simply fall off now. I'm taking that as a sign they need to be replaced.
Do you periodically replace your nocks or wait until you notice a problem?
Might need to reserve your string. I have not had to replace nocks, but sometimes their will be a slight indentation in the serving which will cause this.
Danny
Usually when I see they are Cracked, or I see them Spreading Colored Plastic when my Arrow bounces Back!!
When my nocks get loose on the string (assuming the serving is OK), it is a sign they are about to break. I've had enough break that I don't mess around anymore. I retire the arrow until I can replace the nock.
Dave in Ft. Collins, CO
I replace plastic arrow nocks when they break or fall off the arrow. Try a new nock on the string -- if it seems loose, either replace or re-serve the string.