Potato - all top and no bottom

I am a beginner-ish.  I have been trying to grow first early Anya and Pink Fir Apple potato for the last 2 seasons in raised beds. Can anyone shed some light on why each plant puts on a huge (more than 8 foot long) top like a triffid, yet there are hardly any potato tubers below ground.  I have tried lifting at 100, 120 and 140 days.  The tops don't like to turn yellow either, keeping their green till the death (or I get fed up and I pull them up).  The only success I had was in 2 tubs where the tops where more limited and there were reasonable tubers at 100-ish days.

Any ideas would be very welcome....
Thanks in advance.
MW

Welcome to the forum 

This is a little odd. Are you feeding the ground with anything? How well are you keeping them watered - raised beds can dry out very quickly?

I agree that PFA has a long growing season, but Anya should be ready sooner, so there may well be something else going on.

I think you have cracked it.  I added in a lot of home made compost and added chicken manure pellets when I was feeding the green vegetables.  Therefore I think I am seriously overdoing the N and I am way low on the K.

That's a plan then.  Third time lucky?
Thank you very much.
MW