Alan, thank you for the answer.
I opened the melodica and haven’t any correlation between gaps and ease of blowing. So I took some pictures:
http://grushetsky.ru/temp/DSC01241.JPG (from the lower notes side)
http://grushetsky.ru/temp/DSC01242.JPG (lower notes side)
http://grushetsky.ru/temp/DSC01248.JPG (higher notes)
Should I try to lessen any gaps? I’m pretty sure that it was easier to blow most of the notes (except a few of the highest and maybe the lowest) before my intrusion.
And yes, I understand that Hohner Student is a low-end instrument for $25, but why not to try to improve it…