When you have a file checked out and are trying to check it back in, you must save it first using "File - Save" (Do not use Save As or change the file name/location), then check the file back into AMP™.
When the file is checked out, it's opening the file from your hard drive location, and when you check it in, it takes that file and re-uploads it into AMP. If you check the file back in without saving it first AMP will prompt you with the following message:
"The file has not been modified. Please make sure your content has been saved to the correct location. If you simply want to unlock the file, select 'Revert' instead."
Reverting a file is similar to checking it in, however it will revert the file back to its state before it was checked out.