Problem with Unix directory permission

I have this problem where I had changed the file permission to allow the user, group and others to read, write and execute. But after somethime, somehow the file permission changed to disable the others to write that file. how could this happen? Help me on this please.

Countless reasons simply too many and diverse to mention in their entirety.
Could be a health check that runs which "repairs" file perms, could be another user, could be a software package that runs locally which alters perms to suit it's own needs....

There are simply too many variables to adequately answer that question.

Make sure you change the permission of the subfolders as well

chmod -R 7**