If then formula stopped working 74 lines in to sheet

I have an extensive spreadsheet I use to track tax savings for a job. We have formulas in the H and I columns of the attached sheet on the first tab. They have been working for months. All of the sudden, the formulas are no longer working and I can not figure out why. I have highlighted the areas where the problem is. If anyone can help me figure out why they would no longer work, it would be greatly appreciated.

The formula in H column is =IF(E4=0," ",IF(SUM(F4-G4)<=4999.99,SUM(F4-G4)*0.075,(SUM(F4-G4)*0.06)+75)) and in the I column it is =IF(E4=0," ",SUM(F4+H4))

When I try that, it only returns a total of $75 instead of $1,350.68 as I would expect the total to be on H74

Also note that " " returns a space. I think you are looking for "" which returns a blank.