Data Validation problems

My question was answered last week about receiving an error message and stopping input when the amount formulated in my difference column was not equal to zero. It seemed to be the perfect solution, BUT it does not work unless the input to calculate the difference column is already entered. I was testing it on a completed spread sheet. Now that we started a new week, I immediately get the error and it will not allow the correct entry to bring the difference cell to zero. I tried checking and unchecking the ‘ignore blank’ box and it didn’t seem to make a difference either way. I will be glad to email someone the spreadsheet if that will help. I may have to do this a different way. Thanks,