This may occur when the Expiration_Date and Last_Date are null for the Account_Activities record, or older than the "Years to Last Gift or Expiration in EOM" setting in TA (under Procedures/End of Month Processing).
For example: if the "Years to Last Gift or Expiration in EOM" parameter is set to 5 years, and if the current calendar year is 2016, it will pull records with a Last_Date as far back as 2011, but anything before that it will not recalculate. Similarly, if the Last_Date is null, it will also not recalculate it. The same applies to Expiration_Date - if it is null or the date > 5 years ago, it will not be recalculated. If either Last_Date or Expiration_Date are within the 5 year setting, then it will be recalculated.
If you want these records recalculated, contact TA Support to have them manually run a recalc of the account activities records for the account.
Also please note that the End of Month Account Activities update does not recalculate the Account Activities from scratch; it just updates certain fields. So it will not, for example, recalculate the Expiration_Date.