In Excel 2007 Why Doesn't Locking And Protecting Cells Work

I've created a workbook with about 15 worksheets and prior to sending to the customer I want to lock the cells so that they can't be changed without a password. So, I first select the cells I don't want to be altered and lock them. Then I go to "protect worksheet" and after checking the box that allows people to select cells in the locked section (selection of sells in unlocked section checks automatically) I enter a password twice and the worksheet is theoretically locked.

I can still change cells in the locked area. It didn't work on any of my worksheets. So I went back and unprotected the sheets and then selected and locked the cells TWICE, after which I protected the worksheets again. This time some, but not all, of the worksheets had locked the cells protecting them from editing. I then tried to go thru the same rigmarole but locking the cells three or four times before protecting the sheets for the sheets that were still unlocked, but that only worked sporadically as well. Sometimes I had to dis-allow selection, lock the cells in the sheet, unprotect it, lock the cells again twice, and then check the select box and protect the sheet AGAIN. That FINALLY worked.