This code has been working perfectly for months, but suddenly it stopped. The last time the email template was used was 2/24/14. I have verified that there have been conditions where it should have been sent since then but hasn't. Any ideas?

global with sharing class NPSEmailSenderBatch implements Database.Batchable<SObject>, Database.Stateful
{
    public String query = '';
    global Set<ID> contactIDsToSendEmailTo;
   
    public NPSEmailSenderBatch()
    {
        query = 'Select ID, Name from Account';
        contactIDsToSendEmailTo = new Set<ID>();
    }  
   
    global Database.QueryLocator start(Database.BatchableContext BC)
    {
        return Database.getQueryLocator(query);
    }
   
    global void execute(Database.BatchableContext BC, list<sObject> scope)
    {
        Set<ID> accountIDs = new Set<ID>();
        for(Account a: (List<Account>) scope)
            accountIDs.add(a.ID);
       
        Set<ID> accountIdsToSendEmailTo = new Set<ID>();
        for(Contract ctrct: [Select ID, AccountID, Subscription_Start_Date__c
                             from Contract
                             where AccountID in: accountIDs
                             and Contract_Status__c =: 'Active'
                             and Subscription_Start_Date__c !=: null])
        {
        //send every 6 months and 12 months from subscription start date
            if(
            ((ctrct.Subscription_Start_Date__c.addMonths(6).month() == (Date.today().month()))
            || (ctrct.Subscription_Start_Date__c.month() == Date.today().month() && ctrct.Subscription_Start_Date__c.year() < Date.today().year()))
                && ctrct.Subscription_Start_Date__c.day() == Date.Today().day()
            )
            {
                accountIDsToSendEmailTo.add(ctrct.AccountID);
            }
        }
       
        //if we have qualified accounts
        if(accountIDsToSendEmailTo.size() > 0)
        {
            for(Contact c: [Select ID, LastName, FirstName, Key_Contact__c
                            from Contact
                            where AccountID in: accountIdsToSendEmailTo
                            and Contact_Status__c =: Constants.STATUS_ACTIVE
                            ])
            {
                if(c.Key_Contact__c != null && c.Key_Contact__c.contains(Constants.CONTACTOBJ_PRIMARY_CONTACT))
                    contactIDsToSendEmailTo.add(c.ID);
            }
        }
    }
   
    global void finish(Database.BatchableContext BC)
    {
        if(contactIDsToSendEmailTo.size() > 0)
        {
            List<ID> contactIDs = new List<ID>();
            contactIDs.addAll(contactIDsToSendEmailTo);
            Messaging.MassEmailMessage mail = new Messaging.MassEmailMessage();
            mail.setSenderDisplayName('Client Care');
            mail.setTargetObjectIds(contactIDs);
            mail.setTemplateId(GenericServices.getGeneralSettingValueForKey(Constants.NPS_TEMPLATE_ID));
            Messaging.sendEmail(new Messaging.MassEmailMessage[] { mail });
        }
    }
   
    global static void startCalculation()
    {
        NPSEmailSenderBatch cesb = new NPSEmailSenderBatch();
        Database.executeBatch(cesb);
    }
}