Exel summing problem

I have recently been experiencing an intermittent problem with summing using auto sum and with the basic sum+ then +what ever cells I want to sum

Answer I get is 0.00

I have tried reformating, forcing the format, clearing cell contents - but have not found the solution

Whats infuriating is when I copy a correctly aurosummed cell and try to insert it into the problem cell I get the original cell's total

What can I do - any suggestions greatfully received