If your Ironman save is no longer eligible for achievements it means that something has caused the game to change. It can be several things, from a game update to newly installed mods to saved game file corruption caused by any number of possibilities. Regardless of the cause we cannot make them earn achievements again.
Once a save file loses Ironman status it's impossible for us to revert this, and you will need to start again. You can read more about the various criteria for our games here: