We're currently experiencing an issue that occurs if you edit an activity using an older version of an iPhone app. If you're running a version of the iPhone app older than 4.2 and edit an activity in any way (change name, bike type, etc.) then the achievements will be removed.
If you're seeing this, please update to the newest version of the app and this should prevent the problem from occurring in the future. If you are unable to update your app due to OS limitations, we apologize and we're looking into a fix.
For the time being, if you experience this, you can manually fix this by going to the activity page on the website then making your activity private and public again.