Sometimes fish will have their growth rate reduced as a result of stress from conspecifics, especially submissive males of fiesty-agressive species, almost as if they are trying to stay "under the radar" of the alpha male.
It can also be other factors like an internal infection, being outcompeted for food, not enough water changes, not a high enough frequency of feeds (the latter two being more of a general tank group issue than individuals).
My own personal observations of slow growth rates have happened with Lionhead Cichlid (Steatocranus casuarius) youngsters. The first batch of 13 reached ~6/7cm SL after 6 months in a Rio240 with standard water changes, yet last years batch of ~50 were still struggling to reach that size after 9 months with more 50+% water changes and more feeds.
