I once asked a question about this on 'Gardeners' Question Time' so I always remember the answer. It was just about celeriac but the advice was to tip my entire compost heap over a square yard of garden and dig it all in before planting.
My experience in the 15 years or so since tells me that celeriac have to have plenty of moisture and very rich soil indeed, to make big roots. They must never get a check when growing and must be kept moist and planted fairly early in the season.
Remember they are a member of the celery family and that too requires lots of moisture and rich soil.
I know many people have problems with swedes, and I don't grow those, so perhaps somebody else can tell you what to do about those.

Quote;i feed them with chicken pellets.

And this didn't help, as it added too much nitrogen which promoted leaf growth not root growth!