Help! All plants stalling/slow growing

Hi folks,

Complete beginner here, getting frustrated with trying to grow something that should grow like a weed!

My seeds (Royal Queen Seeds automatics) all germinate well (damp tissue method), and when their taproots are 1-2 cm long I plant them out in small pots, approx 0.5-1 cm deep. Compost is mgc24 “bio-hanf” peat-free brand. They grow, possibly a bit stringy, but maintain a nice green colour, on the windowsill. When they have another couple of pairs of leaves, after their cotyledons, I move them outside, to a SW facing patio with a wall behind them. It’s been warm, but rarely >30 C, and I’d say it’s been sunny enough - the rest of the garden is fine….

Then the plants stall - they get a few more leaves, never show any interest in e.g. the stalk branching, and turn a uniform paler green. They do not grow taller than approx. 10 cm, with my oldest plant from late May having started to grow a single tiny bud (is bonsai cannabis valuable :-) ).

The water here (Berlin) is quite hard, but still tests as having a pH of ~7. I’ve also tried watering with rain water and acidified (citric acid - it’s what’s in the Biobizz pH down) water.

I’ve tried Root Juice, Fish Mix and Alg-a-mic fertilisers from Biobizz, nothing seems to make any difference, and with such small plants it’s difficult to not overwater. I’d also expect the compost itself to still be providing enough nutrition for such young plants?

All help/suggestions gratefully received - thanks! Sorry for lack of a picture - for some reason it’s not letting me upload anything.