Make sure you are comparing the same variety of bok choy to your own. And sometimes genetics vary between seed packets that are labelled the same.

There are a lot of variables beyond variety as well. Oxygenation and bacterial balance in the root zone is important; it is related to reservoir temp as well. Look at your roots - you want them to be thick, bright white, and fuzzy. Thin brown roots that fall apart and detach from the plant easily are what you don't want.

.I have not grown either bok choi or buk choy, but it is a Brassica vegetable grown almost like lettuce and if I did it I would be sure there was sufficient iron and give it a similar solution to lettuce except more concentrated since rapa has quite an appetite and like broccoli has a high nutrient demand to produce all that nutrient rich content. If you run the pH around 7 you may limit growth depending on how good your control is, because over 7-7.2 iron becomes limiting and manganese is also slower. Since other Brassicas do fine at pH 6.5 to 6.8, I would do that, use a lettuce formula but instead of a CF of 16, I'd be using around CF=24 in comfortable weather and hope that sped up the growth which I expect it would for medium and bigger plants. Of course if it is hot as Australia there and your plants are transpiring lots of water I would cut back on the CF to the CF=16 level you are using to try to strike a balance between the increased plant uptake due to the heat and my target in regular weather around 22C.

Like I said though I haven't grown it so these are just mullings. Good luck.

Thanks to you both for giving good advice I will try to achieve what you have said.
It is possibly the aeration of the nutrient solution in the internal drip system which is incorrect, however I will try all your recommendations.
Bob

After this advice I have excellent growth success with Bok Choy in my DWC at a ph of 7 and an ec of about 1.5 with fresh produce available most of the time.