Disappearing formulas!

I have a spreadsheet set up with an 'IF' statement containing a formula in the final column of each row that gives me a cumulative total of transactions to date, i.e. it adds a value in the immediately-to-the-left-cell to the immediately-to-the above-cell, but only if the immediately-to-the-left-cell is not empty. The formula is replicated in many rows ahead of where the last transaction has been recorded. This has been working OK since I started using Zoho — until now. Now the formula mysteriously disappears whenever I add a new transaction into the cells to its left, so I no longer get the cumulative total calculated and displayed. However, the formula remains in the cells in subsequent rows! Help!