Scheduled reminders on membership end date not sending
I posted a summary HERE just yesterday that should provide you with way too much detail as to the cause of the issue, but by far the easiest way to fix the issue for existing records is to enable repetition (being sure to enter every 1 year until 0 days before membership end or the scheduled job will get mad and won't run). 

In the "Current Behavior" section of the same post, you will find a detailed description of how to test to make sure schedule reminders will continue to work - if you don't have access to the database and want to be completely sure that the renewal reminders will send when expected, the next easiest option would be to create new schedule reminders and disable the old ones.

Hope this helps,
Tamar