Users imported, now gradually expiring?

Last week I imported a CSV of around 180 users in to S2member. The CSV included payment gateway (paypal), the unique Profile ID (I-P1XBBYFxxxxx) for each user, the date their last payment was made, and their EOT (1 month after last payment date).

I presumed S2 would look to see if their recurring payment had been made on the date of their EOT and renew their access for another month but as I check daily, users are dropping off and being expired even though they have paid for extra access.

What could be going wrong here and how can I stop S2 expiring members?

Thanks