I'm assuming, first of all, that you checked your xml and Database log files for error messages. If there were any other XML errors in your file (say, listing the bonus for Flood Plains under the Terrains table instead of the Features table), then it'd cause the whole thing to fail. You can't just trim all of those bits out of your post and expect us to find a problem in the little piece you posted, because the error might not BE in that piece.
Second, art definitions are managed through a non-GameData XML file, and they're all combination-based. As in, "improvement X on terrain Y" or "improvement X on feature Z" or "improvement X on resource A". If your improvement is supposed to be placed on exactly the same terrains, features, etc. as Farms (or a pure subset of such) then this part should still work correctly, but if there are ANY new combinations involved or allowed, then you can have art issues. Try to place an Offshore Platform graphic on top of a Uranium graphic, and both graphics will blank out, leaving the appearance of an unimproved tile. Unfortunately, for some reason the game won't accept modifications to this particular file, even with VFS, which is why we can't currently add new terrain/improvement/resource graphics in a mod.
(Note: you CAN do certain "invalid" combinations and still have it work. For instance, you can place a Mine graphic in the water, and it'll just look half-submerged. The Water/Land thing in general does this, where resources and improvements meant for land can be drawn in water and vice-versa, because the difference between water and land isn't actually a terrain type, the two are different Plot types instead. Different rules for compatibility apply.)
There's also an Era-related bit, where some improvements use a different graphic in early eras than in later ones. This is tied into the art definitions themselves, where you might list "ART_DEFINE_MANUFACTORY", but what the game actually stores internally is "ART_DEFINE_MANUFACTORY_EARLY" or "ART_DEFINE_MANUFACTORY_LATE" depending on era. So if you're modifying one of the units (like the Worker), buildings, or improvements whose art depends on era, then you need to make sure the flag designating it as such keeps the same value as the one you're basing it on. This also applies to the Culture Style flag for improvements and units; there are four Landmark graphics, and four Settler graphics, with one for each of the four art styles (Asian, American, African, European). You can use this to your advantage; in my mod, the Monolith improvement uses the African Landmark graphic only, regardless of which group your civ is actually in, so that most civs will see the two with different graphics.
Beyond that, I'd have to see the actual files. The problem could be in the Improvement, the tables related to the improvement, the Build action or its tables, or even the features/terrains you're linking it to.
But again, I've successfully added a dozen or so new Improvements in my mods, all of which use existing graphical art definitions, and all of which work. So obviously, it CAN be done.
