Usually, it is one of two reasons:
1) Dirty cell. Remove cell and inspect for calcium deposits. If dirty, clean. I use bamboo skewers to remove calcium deposits rather than soaking in acid. But if that doesn't work, it make need a soak. I use 1:4 14% acid:water ratio for soaking but only when absolutely necessary.
2) Failing cell. As SWG cells ages, it degrades and the salt level reads lower than it actually is due to lower current. Many pools stores can test cells to see if they are working properly or not.
The cell is only 24 months old but if you have running it near 100% for those 24 months, then it could still be failing.
What is the average percent setting you use and what is the pump run time?
How big is your pool?
Side note: it would be helpful if you had a signature with all of your equipment:
www.troublefreepool.com