The terraforming improvements you can build depend on the soil quality of the planet. The very last terraforming improvement should be the ultra-terraformer which allows the same tile-quality as the first improvement (soil enhancement) to be used, but this time you can build that multiple times (but only on the soil enhancement-type tiles).
The other terraforming improvements allow lower quality tiles to be used, but it can happen, that you used up all possible quality-tiles already. To check, you can use the "biosphere" improvement, which allows any tile, this really means any tile to be used (but you can only build it once).
So if your planet has no tiles left, that can be used with the improvements you can still build, they remain in your improvement que, but you can not build them (happens especially on very low quality planets and extreme planets). The best way is actually to safe up all the build-once-improvements and compare with the soil enhancement improvements to choose the tiles, you can get with these (for the others of the soil enhancement you can use the ultra-terraformer later). Doing that, you can see that sometimes, the available tiles for terraforming don't change if you switch e.g. from terraformeing to habitat improvement (which should allow to improve lower quality tiles), which means, that there are no additional tiles available on this planet.
