Working hours attempt URGENT

In a bit of a pickle, I thought id solve this by today and am running myself in circles...

I have been working on a spreadsheet that requires me to calculate the number of working hours (9-5) spent on a number of tasks that can either be resolved in a day or span multiple days/weeks/months. Oddly, my attempt is correct in some instances (marked green) but incorrect in other (marked red).

I would be eternally grateful if someone could dig me out of a big hole and provide the correct formula.

Many thanks

Max