Why haven't I received more Lucky Eggs since level 12?
In conjunction with luser droog's answer, to answer your question in regards to levelling, you receive a Lucky Egg (along with other rewards) when you hit levels:
- 9: 15 Poké Balls, 10 Potions, 5 Revives, 3 Razz Berries, 1 Lucky Egg
- 10: 15 Poké Balls, 10 Super Potions, 10 Revives, 10 Razz Berries, 1 Incense, 1 Lucky Egg, 1 Egg Incubator, 1 Lure Module
- 15: 15 Poké Balls, 10 Super Potions, 10 Revives, 10 Razz Berries, 1 Incense, 1 Lucky Egg, 1 Egg Incubator, 1 Lure Module
- 20: 20 Ultra Balls, 20 Hyper Potions, 20 Revives, 20 Razz Berry, 2 Incense, 2 Lucky Eggs, 2 Egg Incubators, 2 Lure Modules
- 25: 25 Ultra Balls, 20 Max Potions, 15 Revives, 15 Razz Berries, 1 Incense, 1 Lucky Egg, 1 Egg Incubator, 1 Lure Module
- 30: 30 Ultra Balls, 20 Max Potions, 20 Max Revives, 20 Razz Berries, 3 Incense, 3 Lucky Eggs, 3 Egg Incubators, 3 Lure Modules
Source
You may get some at higher levels, but I'm not sure when (or even if you can surpass level 30).
As luser droog suggested however, if you want more eggs more frequently, you're better off cashing in on the defender bonus to buy them, or paying for micro transactions, depending on how desperate you are.
Get in some gyms and buy one!
Seriously though. If you can't hold on to a gym then just cash out right away. 1 gym X 8 days = 80 coins == 1 lucky egg.
If you can hold a gym for a while, then hit a few on your poke-walk and cash-out when you get home.