Can't' research lvl7 elite unit (bombers)

Hi,

I am trying to invent lvl7 bombers. I used gold to get the blueprints (4/4) but the game tells me that I don't meet the requirements. The day is already past the 22nd day (others already have it) I do have all the previous levels researched.

Could I learn why this is the case? the artifacts were a little expensive to be bought "for nothing".

Thanks, Jas