I was able to identify the problem why the Bluedream grows so poorly. Some leaves had covered the substrate and mould had developed there.
I removed these leaves, loosened up the substrate and installed another fan. It actually took 7 days for the pot to become noticeably lighter, it was practically standing in water and hardly any air was getting to the roots.
The switch to flowering fertiliser also came too late. So all mistakes on my side that were caused by too much watering.
Nevertheless, they are continuing to develop well and I hope the end result will be reasonably satisfactory.