Celeriac fail!

Last year we tried to grow celeriac for the first time. I raised some lovely healthy little 'Prague Giant' seedlings at home, planted them out in a row on the plot around the end of May, they produced some really healthy and vibrant plants with a delicious celery-like aroma and classy purple stems and... nothing. I watched those roots, I waited, I watered, I waited some more, and they never swelled. I'm pretty sure they can't have wanted for water (especially with mycorrhizal fungi applied) and they should have had plenty of nutrients too (and everything else in the same bed certainly did well).

I cleared the row in disgust in mid-November, and there was nothing but a tangle of roots beneath those lovely thick stems. I've read since that they do all their swelling in the late summer and autumn, so maybe, just maybe, they were about to surprise me just as I gave up on them, but c'mon... mid-November? Surely they should have shown signs of growth by then?

I'd like to try again but it seems sort of futile unless I do something differently - but I really don't know where I went wrong, and it's a popular variety that plenty of other folk succeed with. This year I will make sure to sow them early and plant out as soon as possible, and make sure they always get plenty of water, but if you have any other ideas or suggestions for better success, please do tell!