Dormancy on a membership considers the entire membership, so any activity on any of the sub-accounts will suspend it from dormancy.  If there is more than one account with recent activity, all accounts would need to be adjusted to remain in dormancy. 

Make sure the Last contacted date hasn't been changed on the member's account.  This is used to record when a member contacts the CU about keeping their account active, without performing an actual transaction.  (Use Tool #15 Update Membership Info.)

Your dormancy configuration might also take other things into account besides transactions, such as activity on other household accounts, multiple memberships per SSN, etc., so review your configuration for other conditions that might be in play (use Tool #950 View Dormancy/Escheat Configuration, or, if you have access to the config itself, Tool # 346 Dormancy/Escheat Configuration).