Question : Excel Cells with Formulas not updating when inputs change

Hi - I'm a long time, intermediate Excel user.

Spreadsheet is small but dense with a lot of cell naming & basic formulas including:

Cell A180:          =A2
Cell A181-186:   [manually entered single digit/1-decimal numbers]
Cell A187:          =SUM(A181:A186)
Cell A188:          =A180/A187

PROBLEM:  when a number is entered into a cell in range A181:186 (i.e. enter a new number then press Enter or Right Arrow), cells A187 and A188 do not always update - they sometimes do & sometimes don't.  It does update when I click in the cell that hasn't auto-updated, put the cursor in the middle of the formula in the formula bar, and press enter.  

Question edit/update: Just found there are other places in the same spreadsheet where other =SUM(Range) formula are not updating when inputs are changed. Similarly, when I put the cursor in the formula for the cell that didn't auto-update, and press enter, it updates.

I'm fearing corruption.  Help please!

Thank you.

Answer : Excel Cells with Formulas not updating when inputs change

You must have set calculation to Manual. Change it to automatic.
http://www.ehow.com/how_2248633_switch-manual-calculation-excel.html