Pivot table behaving oddly

Hi,

I have setup a pivot chart that is referencing a dataset referring to number of minutes spent in various modes of exercise across days of the year. The pivot table behind the chart appears to be picking and choosing when it wants to sum the data I have requested it to- and I need some help figuring out why!

Refreshing the table does not fix the problem. It clearly knows the data is there as when I switch the value field settings to 'count' it reflects the data is there as it is counting it, but for some reason refuses to sum the data when I put the value field settings in 'sum' mode. Instead it returns a 0.

The data in the original table is inputted through drop down (data validation) menus so I don't think the format of that is the issue. Any ideas are greatly appreciated, as I am currently convinved I have a gremlin in my pivot table!

Cheers

Emma