Two pivot fields in column - not displaying one of them

Hello,

I'm sure i'm doing something very obviously wrong but a standard pivot seems to have eluded me.
Attached is my pivot, I want to view the 'month' and the 'cost type' as columns so it displays like this:

Month
Cost type 1 Cost Type 2 Cost Type 3

... and so forth. But cost type is not appearing in the pivot. When I move it above Months so it looks like the below it works:

Cost Type 1
Month 1 Month 2 Month 3

What am I doing wrong???