HI,
    This can sometimes be a problem, if you look at the read me file in your plane, it will possibly say something likes "requires Spitpak add on" or something similar, then all you need to do is look on the site it says to download from,and install it into yor various game files, also what you may have downloaded could be one of a series, ie Texan 3 out of 5 , so you will find that Texan 1 has all the required parts, and yours is the "light" version so to speak,ie requiring the "extra" bits from download 1, hope that helps