If you have the MC Cheats add-on, you can change the current value to whatever you want: click on your sim and select NRaas > MC > Advanced > Lifetime Wish: Change Tally. This won't fix an ongoing error with the tally itself though.
Can you tell what items are no longer being counted? For example, maybe one high-value item is subtracted, and it turns out you or your sim did something with it that would explain the change. (Or it's not counted until you take the item out of inventory and place it.) I've also heard that traveling tends to reset the tally, for whatever reason. It might turn out in the end that it's easiest for you to have your sim complete the wish in one spurt rather than over a long period of time.
Not the point here, but it's always bothered me how the wish itself is called "Possession is Nine Tenths of the Law." The legal term is "possession is nine parts of the law, but pop culture gets it wrong so consistently that it seems like a lost battle at this point.
