Mesh elevation issue

Hey everyone, looking for some guidance on an issue I encountered recently. I have a multitude of freeware and payware addons for FSX, but recently, I started noticing that while flying into certain airports, ATC will give me directions to descend and maintain 0 until established, and the airport might not even be at 0 MSL. So the AI aircraft won't know to ignore that and they land short of the airport by 5 miles and then taxi in (even over water). Obviously the program has corrupted the elevation of certain features but I cant find the problem. Thanks in advance for the help.