They germinated well and started to grow and then did not get much bigger.
Snap!
Exactly the same symptoms I have suffered from this season!
My findings....the compost was too acidic!
As I said I had problems as described above and tried all sorts of things to rectify the situation in the end after seeing the state of the root systems of dead or poorly plants....they had not developed!(see pic)
I suspected the compost pH but did not have a decent pH meter to prove this, so I searched high & low on Amazon and finally plumped for one (very few had 5* reviews)
As soon as I stuck the meter into the compost the needle flew over to the acidic side of neutral and on one occasion I got a reading of 5 although to be fair most were between 6 and 6.5.
In fact, I tried my old meter which I have never seen the needle move out of the 6.6-7.7 zone, this time it did!
It was then I decided to start mixing my own compost again! So again I went onto Amazon and bought some John Innes base fertiliser to add to my existing compost along with a regulated quantity of lime!
Touch wood! The plants that I have re-planted into my own mix are surviving but due to the initial check in growth they are not up to the standard I would like.
I mentioned the following in a previous thread that on the compost bag in fairly large print
"That this compost is not suitable for commercial use" Why I asked myself!
I got on to the producers as opposed to the suppliers, and they said that I was using a "Retail" mix rather than a commercial mix the recipe is quite different!
So not a level playing field then! Something I have always suspected since the emergence of Peat free/reduced composts!
Now for my opinion and it is this: The commercial stuff has the cream of the peat and the retail stuff has the dregs!
So with sphagnum based peats which comes from 'bogs' rather than 'heathland' the commercial stuff gets the actual moss but the retail stuff gets the silt from the bottom of the bog!
Now as we are often told peat stores large quantities of C02 which is (in my opinion) washed through into the silt at the base of the bog turning anything that is in contact with it into an acidic material i.e.......................
the stuff used for RETAIL compost! Hence, the warning on the bag.
Now back to your Brassicas!
Brassicas need an Alkaline soil/compost and do not perform well in soil/compost with a pH of less than 7
So if my comments above have any truth in them then this is why your Brassicas are not performing as well as you would like them to!
p.s I must emphasize that all I have written above is my personal opinion and I have no real proof if I am right or not, that is for you to decide!