I get on best with potato's in containers, usually plant 3 in a pot of 48cm dia, at first with the pot about half full of compost and , like yourself, some chicken pellets (quite a few). When they sprout and get big enough they get more compost and more pellets.
Always kept damp as they get scab if too dry, and the tuber needs moisture.
Potato's need feeding, potting compost will not last long, and not sure how much pelleted manure you used. I used to put on the surface a full pot that of about 3-4 inch dia. Full scoop and on it went, twice over the time they are in the pot.
Notice that people are saying they have flowered so I harvested. What is the connection? Only started hearing this over the last few years and last year I drove daily past a large field of potato's. They were not harvested when in flower nor any time soon after. Same for the field that I walked alongside at lunchtime. Just if commerical growers leave them I suspect there is a reason, like size and/or yield.
I would have thought that the process was sprout, grow, flower, create seed, then create tubers for the next year. As the tuber is the potato? (all guesswork
) Would removing the flowers cause them to create more tubers? Their chance of passing on their genetics via seed is plucked so making more tubers means a bigger chance of managing it the next year.
Anyway my guess is slightly insufficent water and food. Maybe's not by much but it wouldn't take much. Also last years posts like this, are your expectations realistic. Last year it seemed that one person planted a potato and expected sufficent to feed them for the rest of the year. Think they expected about 5kg. I would estimate 4-5 from a plant and one or two of them would be smaller then the others.