Stunted Agapanthus

I bought three bare root Agapanthus that were delivered around February this year which I potted up in a largeish pot in general purpose compost with some stuff for drainage at the bottom of the pot. I then left it in the cold frame for a while due to the miserable spring and I did know they can be a bit flakey with the cold. One sent up leaves in the spring but it seems to have stopped at around two inches and just stuck there. Just noticed another of the roots has done the same quite recently, I assumed the other two had died due to me being a bit rubbish. The variety should be Blue Danube.

Any ideas why this has happened, maybe just need to leave them to feed up this year and maybe get a bit more from them next year? Or is it worth feeding them with something as well.

Possibly the roots had dried out somewhat when you got them? Also as said, could just be babies.