Same issue here. Did the initial flight and have had nothing since. I fly in California, Arizona, New Mexico, Europe, Portugal and Canada. I have a decent coverage area opened up and not one fire mission in over a week.
Just got my fire fighting company hoping it would spawn stuff, nothing.
This whole coverage map thing is a great concept but makes zero sense for fun and the map itself shows in GREENLAND that in the winter its a hot spot… there are no trees in Greenland. Just generate missions like every other specialty. I was very excited for fire bombing and its been a horrible experience grinding up to it in a Skyhawk for hours to waste a bunch of cash and not be able to do it.