There are a few possibilities for rollover pages not being properly credited:
Rollover pages are not allowed if you have the free fifteen page per month plan.
If the printer is not left turned on (automatic standby mode is OK) it may not properly communicate usage. Make sure the printer is left turned on, it will automatically go into a low power standby mode but still be able to communicate with the server.
You can check the Instant Ink dashboard to make sure the printer is properly online.