Mail Not Sent - Queue Not Cleared

Any suggestions on steps to troubleshoot this problem?

I am using Subscriptions 6.x-1.0 on Drupal 6.14 and e-mail messages are being sent.

Entries are created in the {subscriptions_queue} table.
Entries in queue table are not removed when cron runs.
No errors reported in dblog
syslog module disabled, I am on a shared host.

Mail sent through Contact, Private Messages and other modules is working fine.