I received my first increase at 4 months.
And they definitely have given auto increases. You can search the approval forum by typing "discover auto" and results will populate.
It's not a given and profile dependent.
Typical results though show if it's going to happen it usually creeps up within the first 2.5-6 months but, sometimes it could take years to happen. There's no method to the madness with Disco + CLI's.
... When did you get your first auto CLI?
What's your profile look like?
There is absolutely no guarantee of a CLI--ever!
The reason could be a million and one things or no reason at all. But the most probable reason in your decline letter will be "not enough experience," which makes sense. It's only been 3 months since you've had the card.