I just want to get a correct total!

I am very new to Ubuntu(7.10) and thus to OpenOffice, I am over 60yrs and just want to have a budget spreadsheet that is easy to use. I have set it up as it was in Excel on old pc..and for the first few entries had no issues. Now I am not gettint Totals when I try to add/subtract new numbers from the old totals. I am using the sigma SUM button, I have autocalulate and auto input ticked. Please, in simple terms, what am I doing wrong? If I retype the previous figure and then use the SUM I get a correct total, but otherwise it only recognises the new entries. Help! Please!