Do you have write permission on the file?

Is the hex file not saving with those editors, or is the game just not showing the new value once it's loaded?

The file could be digitally signed. By editing you change the hash value that will be computed when the file is checked for integrity. Just a wild guess.

I don't know these programs. Some hex editors are weird about editing, I use bvi to edit and it works pretty good for me.

I had similar frustrations with a game. It's very easy to make mistakes while you're a beginner in editing such files (I don't know if you are).
One advice is to make sure to keep the data the same length.

If that doesn't help, observe the file's structure a bit more. Maybe it uses a checksum somewhere for the data you want to edit, or it is just stored elsewhere and you were editing the wrong thing.
Make a save. Make the data to change (in the shortest time possible) and make a new save. Compare these for what have changed.

But also, what is your problem?
Does the value just don't change, or the save becomes corrupted?