In real Life, markets eventually get saturated. For example, USA has had Chelsea, United and Liverpool pretty much take over already over a decade, City are relatively newcomers and thus wont get the same fanbase that the others have. In game I think it reflects that, although I dont think this was intentional. Shirt Sales should rise eventually, If you play over 5-6 seasons, you will see the rise.
Feeder Clubs will not be affiliates for profile, You will need to ask your board for an affiliate with merchandising revenues as the main point. For this type of affiliate, you need to check for population, league reputation (medium to high is good), nation reputation and ranking.
In theory, countries like India and China should be easily achievable, but seeing as how Indian leagues themselves barely get spectators, I wouldnt count on India as a merchandising Affiliate seeker.
China and USA are the two main markets as per the game.
