It seems to fluctuate randomly giving you no actual idea whether you are close to reaching 10 stacks or not.
Thank you for reporting the bugs you find, and helping the developers improve the game for us all. This is not the right place to report bugs though, post any bugs you find through the in-game Help menu.
From the Game Menu, click Help and Submit Bug. Follow the guide lines listed, to write up your bug report in details and help out the teams investigating your bug.
Have a few seconds to spare? Let me know how I’m doing!