Syncing is seamless and automatic between any compatible device (including the website, www.carbmanager.com) signed into the same account.
If your data or Premium status is not in sync between your iOS device, Android device, and/or the website, it means that you are not logged into the same account on all devices.
Open both the app and the website, go to Settings > Account Settings, and see which email address is being used by both versions. This email needs to match exactly. If either does not, log out of the incorrect account, and log back in to the correct account.
One common problem that users experience is they skip the signup process on the app, and then later create an account in the website. Remember, data and Premium status will not sync unless you are signed into the same account, and in this case, you won't be using the same account.
The easiest fix is to find the account that has no or little data logged to it, and then delete that account (Settings > Delete Account & Data). Then, re-create the account on the device where you have logged data (Settings > Create Account), and sign back in to this account on all devices.