non growing San Pedro

[splosh]

Any idea what to do with my San Pedros ?
I split 2 big pots last year, to make about 10 new plants. Half are growing fine, and very green. The other half ( same direction, same sun, same water, about 10' away ) have not grown 1" the whole of last year. They are also very yellow. Poor things are just sitting there doing nothing. The others have grown a lot, and produced offsets.
I figure it has something to do with the soil. Tried all sort of fertilizer. Soil is a bit alkaline, so been watering with vinegar. PH around 5.5.
I was told by person at specialist cactus place, that San Pedros need a lot of nitrogen, which is odd. I put in a bunch of blood meal just before the big rains last month ( I'm in Los Angeles ) was expecting to see them green up. Nothing, still just sitting there.
You can see the stark difference in the pics.
Thanks in advance
Steve