Why are my plants growing so slowly

[vktr]

I plant the seeds into a potting mix soil, feed them with balanced fertilizer a couple of times as they grow, and then transplant them into beds or bags with added (once a year) compost. Minimal effort, no problem whatsoever. The main thing is not to overthink it I think

 

[StoneCarver]

So the coco fibre based medium I used to germinate them must have been lacking some essential element.

I'd wager all essential nutrients are lacking. As Kushtaka stated above, coco coir is really just physical support for the roots to grow on. It doesn't feed the roots. Sometimes, it can be very salty; so its a good idea to rinse it real well prior to using. Then amend it real well with nutrients. One trace mineral supplement I like to use is Planters II. Its geologically interesting stuff.
Of course, I mix in compost too and water the seedlings with a Miracle Grow solution.