If the missing account has the same login credentials as the account already linked, follow these instructions to add another sub account:
From any page in the app tap on the Settings Gear “⚙️” in the upper left corner.
From there, tap on Linked Accounts "🏦”
Select Add Account just beneath the bank name
Tap Continue & Enter your credentials
Check the box to give Plaid permission to link your sub account.
If missing account has different login information, you will need to add as if it is a new account. The way to do that is similar to above, but you will use the "add institution" button instead:
From any page in the app tap on the Settings Gear “⚙️” in the upper left corner.
From there, tap on Linked Accounts "🏦”
Tap Add Institution at the bottom of the screen
Type your bank's name and when it appears
Input the credentials that you'd use to access this account online