Turnips not forming

Hi there!

So, five years at current home. First year I sowed turnips very successful with good roots. Every year since - nothing, just long, twisty tapering roots that hang on the surface or very small balls. It's decent soil, emptied the contents of my compost bin on to it a couple of years ago. I always sow directly into the soil at end of March when it's still cool as I know they like the cooler weather and currently they're leafy but no bulb.

I'm just about ready to give up except I know there must be a reason why the roots are forming properly. I read somewhere about adding bonemeal if soil too high in nitrogen but any other tips?

Is it best to just use current turnips as greens, then pull then up, add bonemeal and start again with a new sowing?

My father grows them with no problem at all!

Yours, exasperated in London!