I did it for the first time a year ago. The trick, as far as I could tell, was to have some other interaction with Immigration after you returned to Thailand but before your 90-day is due so that your current situation is updated in the system. In my case, I come back from a trip to the UK in October so the 90-day is due in January. However, my permission to stay on the retirement extension expires on January 1, so I have to apply for a new annual retirement extension in December. Last January, I then tried doing the 90-day online and it worked. I got my latest annual retirement extension earlier this week, and the next (first since I got back) 90-day is due in the first week of January. Whether it will work again or not I will only find out when I apply this time. Maybe the 90-day last January was just a fluke, we shall see.