Need to keep a running annual average of monthly expenses

Hi all,

This worked at one time and stopped a few months ago and I can't figure out how to fix it. Thanks for your help in advance.

I have several rows of expenses for each month;
8 12 Entertainment Animals Eating out Groceries Doctors Misc

Under each column header I have figures that go back 12 months;
Misc

104.78
139.01
40.53
71.56
105.37
203.84

So at the end of every month I delete the oldest row from the top of the spreadsheet and add a new one at the bottom so I consonantly have 12 months of data.

Problem is that the formula won't keep up with the changes. Let's say the formula is =AVERAGE(C4:C15). I want to delete the row that's currently in C4 and add data to C15. As soon as I delete the row at C4 the formula changes to =AVERAGE(C4:C14) and the only way I can seem to get it back it to manually edit the formula for each column.

Not a big deal, I know, but a pain nonetheless. I want to be able to update the data every month and the formula to stay at it's original settings. Any ideas? Thanks and ask any questions that might help.