Pivot not updating

I am working with a excel document that I use every month to compare two sets of data.

I am using a lot of pivot tables but notice that when I update the data and refresh the pivot not all the data pulls through and I have to create a new pivot.

Is there any reason why this would happen?

I want to avoid having to recreate the tables each month.

Thanks

Maybe it has something to do with the Pivot cache?