Why c9 does not save files sometimes


I save the files. it works usually, but sometimes it does not work, and it does not show any sign that file is not actually saved. I can detect file actually is not saved after I create some zip package, deploy to my system. this is very annoying. I could write a very long code and save and see everything is saved without any problem, then close the file. and then, I could realize all my written code is actually gone!! because of c9 did not save it and did not tell me the file actually was not saved because of a mysterious problem which I dont have any clue