Picking Characters

New players from levels 1-5 have a set group of free champions to use (including Garen and Ashe). Once you hit 6 you start using the Free Rotation that changes each week; my assumption is that you were using those 2 until you hit 6 and now they aren't on the free list anymore, and that is intended. Sorry.

But, I have good news! You can obtain Garen, Tristana, and Alistar free through Riot's social promotions found below; https://support.riotgames.com/hc/en-us/articles/202083184-Promotional-Content-Free-Swag-#h2

Just go like, follow, and subscribe to the appropriate pages and follow the instructions. For Alistar the Youtube promotion is broken so you'll need to submit a Support Ticket and request they add it manually.