CBSubs always logs the IPN, even on failed IPNs. If the recurring payment is sending an IPN then CBSubs will log it whether it's used or not. Are you able to confirm using the Access log on your host that the recurring IPN is actually being sent to your server as it sounds like it's not reaching your server at all.
At any rate I've created a bug ticket to investigate this further. In the mean time please use single payments only. Currently I am unable to reach worldpays documentation for one reason or another (appears to be an IP Block for their .com address for US) so I can not review their documentation to see if usage has changed (a likely possibility).
#4465