I'd recommend checking in with your benefits administrator, whom should check in with the Accounts Payable department.
Typically the only time this could occur is if the bill was not paid on time. OR if you do not renew off the statement that is sent to you asking for confirmation (forgot what it's called.) But usually I have never had an issue personally other than the confirmation deal, and my things were covered.
EDIT: I would supply a record of payment and speak with the billing department to be certain they have applied the payments correctly as it still could be part of a 'billing' issue on their end. I worked in HR & Accounting and I did see this occur if BCBS (or other insurances) did not apply things correctly on their end. I do feel it is worth checking on if you want to clear some of this up. (if you have not already done this.).