I don't use GSX nor have it installed.
Then download and install the current version of the Stand-Alone Addon Manager, and see if it makes any difference. If it doens't, download and install the latest full installer for every FSDT scenery you have.
In any case, why you have Couatl, but no GSX ? If you have other FSDT sceneries, there's no reason not to download GSX, since it's totally free to use there.
And not just that: if you don't install GSX, you'll lose some features of the scenery, like Docking Systems, because we removed from the scenery installers and moved to GSX, since we assume that everybody using one of our sceneries will surely download GSX, since it's free so, consider it as an external required piece of the scenery.