I have checked the posts available but I am not sure I can identify the problem with my lemon tree.
The plant was germinated from lemon seeds around one year ago, and it has two siblings who are doing just fine.
We are in central Europe and when we started having a proper summer we put the plants outside, but they started suffering with the heatwave and were moved back inside. They are taking 6 hours of sun these days.
We use citrus-specific supplement (twice a week as described), we recently repotted the plants to improve drainage, we checked the pH of soil and water and it was fine (but the water is extremely hard).
There are no bugs.
What am I missing?