GREETINGS ALL !!

Using APT260 I've created my own basic airfield with a couple of basic textures.

If I sit in my parking area (using spotter-plane view) and do a pan using the hat-switch, or even fly over the airfield; through a section of the compass -  say 270° to 360°, my textures disappear leaving just the autogen!
PLEASE, WHAT GIVES?
WHY DOES IT DO IT TO YOU?   
Can someone please help me out of my misery!   
            Thanks in advance!....