Fluctuations in potato growth

I have planted 3 rows each of Pentland Javelin (early) Kestrel 2nd Early Maris Piper (main)and Desiree (main)
In each case a trench was dug and in it a mixture of my own compost and horse manure which had been stacked for 6 mths.
During the frosty period they were hoed up and protected.
The first row of PJ is excellent, coming into flower, but the 2nd row looks as if it is a month behind.
In one row of kestrel, half is fully grown and the remainder are just small plants.
One row of MP  looks a paler green and not bushy like the other rows.
Desiree were planted later and all seem to be uniform and in good health.
The plants are not dying and there is no suggestion of disease, so the problem has to be in the soil.  Today I gave all the rows a feed of high potash fertiliser.
Has anyone experienced this problem or have any suggestions on what I need to do?

Same here my spuds were planted in the same row and all given the same treatment but are growing at different speeds. Some look close to flowering others are just poking through. My possible variables are drainage which could differ across the bed and the amount of direct sunlight each bit gets. From my crude observations and laymans guess work the bits that get most sun are doing better regardless of which bit drains best. Also I suppose each seed spud is an individual thing so will vary slightly from it's neighbours.

Someone made a good point which I'd not really thought about - the size of the original seed tubers may make more of a difference than we realise

Thank you for your replies. It really needs a photo but I am still on film and it would take too long. The seed are all the same size. with the very small ones (earlies) growing in supermarket flower buckets. I can't compare those-they took a hit with the frost.
I shall monitor their progress after their feed. Perhaps it will correct the problem.