Another possibility is there may be too many *_AFD.bgl files active. I think 200 or so is the maximum that will show up in the Free Flight menu.
Try deactivating other scenery layers containing airfields when activating the Spanish layer.
I separate the *_AFD.bgl files from the other .bgl files and install them in separate scenery layers, e.g. Poland_AFD. I have all the layers containing the airfield *.bgl and *_A16N.bgl files active at the same time, but only the one layer containing the *_AFD.bgl files active for the country or area in which I'm operating at the time in Free Flight.
Kevin