Do you feed all your fish just freeze dried krill? Seems a lack of proper diet/nutrition could be the cause of it staying so small. The addition of the carnivore pellets is good and I'd also suggest a good mixed sinking pellet that contains some veg.
I agree with what others have said about poor genetics especially with cb sens albino or not. But for it to only grow 1" in that amount of time at that age is crazy. Usually they grow pretty quick as I'm sure you seen with your others at least for the first year or so.
Could be that what you are doing for it is the only reason it has survived as long as it has. Maybe in someone else's hands it would not have survived?
Anyway you look at it it's definitely an anomaly. I've never seen one stay that small for that long even in the worst of care. And despite it's small size the pic of yours looks healthy. Maybe it the first of the nano bichirs!