Pivot Chart displaying 0s inconsistantly

[WorthJ]

Hello,

I've run a survey using Email, Phone and Facebook. I'm combining the results into a series of pivot charts, 1 pivot chart for each question, each question has multiple answers

Some of the questions have 0 responses for some of the options(no one on the phone chose option b in question 2)

I've turned on data labels for the table but whether there is a 0 showing when no one has chosen that option is not consistent for every source.

Why is this?

Thanks!