When FSX loads with the PMDG 737 NGX the buildings are missing as are the ground textures. gates, other aircraft and ground vehicles are present.
If I load at the same gate in the Ultralight buildings are present.
If I try a flight from FSiPanel the buildings are missing.
From this description, it seems that, because of the other memory-hungry products you installed, you have (or a very close to) exhausted the RAM available that FSX can use. FSX, as a 32 bit app, cannot ever use more than 4GB of RAM, regardless of how much total RAM you have.
In fact, the 4GB is not entirely usable, the real limit is around 3.7GB and this is much lower if you use DirectX9, because under DX9, video drivers will have to keep a copy of the allocated video card ram so, the more powerful your video card is (allowing to load more textures), the more RAM will "steal" from your system.
The things that will save you memory are:
- Using a less memory-hungry airplane
- Lower your settings, especially the ones related to terrain range
- Don't use HD textures
- Use DX10
Or, a combination of all the above.
This solution is based on your report. You say you have both Addon Manager and the Couatl menu, which means our software is installed correctly. Assuming you still have both menus even when running the PMDG and the other products, the memory exhaustion problem seem the most likely cause.