There are three main reasons for the "no info" label on Grid.
Protected accounts: Protected Tweets are not visible. Only visible to your Twitter followers. We can't fetch the "Last Tweet" data of protected accounts.
Some Twitter accounts have never tweeted yet. They don't have the last tweet date.
We occasionally clean our Tweets database to make room for new tweets by deleting old tweets more than 40 days old. Since the inactive threshold is 30 days, you can safely assume that the users are inactive if there is "no info" on their last Tweet date. (unless the account is a protected account as those accounts are not public)