Scheduled reminders not being sent
In v4.4 and v4.5 there is a known problem with membership reminders. Only one reminder email of each type will ever be sent to each member, not one each year, just one for that member forever. (Unless you have repeat turned on, but I think that is unlikely for membership renewal reminders as that creates another set of problems.)

It means you need to create copies of your existing reminders every 12 months (assuming you have one year memberships).

Things are different in v4.6, so if you are on v4.6 this answer will be irrelevant for you.