Please, please, please help.

This is doin my head in.

I have a crystal report based on a SQL table. The report is based on Branch number and each section is repeated.
i.e.

Branch Number = 1
Field1 = 23
Field2 = 22
Field3 = 35
Field4 = 25
Total Field = 105

Branch Number = 2
Field1 = 20
Field2 = 15
Field3 = 25
Field4 = 25
Total Field = 85


This is fine everything adds up.
My problem is every so often one of the sections will not add up correctly so instead of the above being 85 the total field will display a lesser figure i.e. 70.

The formula would be as follows:
Total Field = Field1 + Field2 + Field3 + Field4

I can't understand why this is doing this , I have set any nulls to be defaulted to 0 but I know this would not effect this as the database field is set to default to '0' anyway.

Can anybody help.