Cylce through report records

[Steve2003]

I have a report that is grouped according to Policy no.s In these groups are values. What I am trying to do within each group is show the percentage difference from the previous value to the next one. I have tried adding vb code to cycle throught he records but it cannot get passed the first record.
Is this possible???
I do not think I can use a query because I only want to show schange per group and not overall.
Please point me in the right direct.
Please any advice will help enormously.

Steve