What aircraft have you noticed this with? If it is just one or two, you might have a mismatch between that aircraft's INI file and the LODs in that aircraft's folder.
For example, in the F8F-1B Bearcat folder, you will see:
[LOD001]
Filename=F8f.LOD
Distance=100
[LOD002]
Filename=F8f.LOD
Distance=250
[LOD003]
Filename=F8f_001.LOD
Distance=1000
[LOD004]
Filename=F8f_001.LOD
Distance=2000
[LOD005]
Filename=F8f_001.LOD
Distance=10000
If the F8f_001.LOD is not present in that folder, or if for some reason there is a typo in the INI file in the name of the longer-distance LOD, the Bearcat will disappear when 250m or more away. The names have to match exactly, or you won't see the aircraft.