Нello, could you comment on this illustrations that was rejected due to technical problems?

IMO, it might work better as an illustration if you added lines to give trees more defined shapes. 

As an example:

Just my 2 cents.

Welcome @Olga_Romanova_art , your water colour trees are very nice.  I do notice that when viewed at 100% magnification, they don't seem to have a sharp focus.  I also see what appears to be artifacts in the trees, but I am assuming that is the texture of paper you used.

Did you scan this image or photograph it?  Scanning may have exaggerated the texture of the paper and created new artifacts.