Trouble restarting a variable once the group changes to 2nd page?

I have to groups in CR 2013, on the details I did a summary for the first group, then for the second one I place a shared variable on the first group and I sum the 1st group value each time it changes. I initialized that var in 0 in the 2nd group in order to restart the var every time the 2nd group changes. That works fine if the 1st group fix just one page, but if is change to a 2nd page I got the var restarted to 0. Can somebody help me please?