What is a non blank cell?

I have a spreadsheet with various details regarding new employees. I have
just added a new function that automatically determines the date for their
probationary period from the start date that is entered for them. I have also
added conditional formatting to two of the cells.

Since adding these extra funtions i cannot insert a new row into the sheet
and a message appears stating that 'Excel cannot shift non blank cells off
the worksheet'.

Why is this and is it possible for me to be able to add new rows at all
without having to delete the new functions?

Thanks in advance