If the account data (metrics) is not showing on the Dashboard at all, it is due to the de-authorization of the linked account. If we get an authorization error for an account, Adbrew stops fetching the data from the Amazon Ads API.
During the de-authorized period, you'll be able to see the account but the data won't be refreshed as per Adbrew's refresh frequency. We retry for 10 days to see if the account gets authorized again, post which we let it remain in this state.
In case you are seeing the data for a few campaigns, but not all. Check which type of campaigns those are. Specifically for Kindle Direct Publishing accounts, Sponsored Brands campaigns may show 0 data even though they are actually running.