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?