NOTE: SLP earning has been removed from Axie Classic and has been moved to Axie Infinity: Origins. If you'd like to continue earning SLP with your Axies, you would have to play Origins instead.
You may check this announcement for more information: https://axie.substack.com/p/phase3
There are currently 4 reasons why you are not able to earn SLP in the game.
Using a recently gifted Axie
An Axie that was gifted from another account will not be eligible to earn SLP from arena for 24 hours after gifting. Just having one recently gifted Axie on the team will make the whole team ineligible.
Using a recently bought Axie
An Axie that was bought from the Marketplace will not be eligible to earn SLP from arena for 24 hours after they were bought. Just having one recently bought Axie on the team will make the whole team ineligible.
You will not be able to earn any SLP from winning an arena match if you do not have any more energy left on your account.
Owning Banned Axies
When you have a banned Axie on your account, the account itself will be restricted from earning any SLP until the Axie on the account was unbanned.