The tension springs in the back do wear out over time and lose power. Either get a whole new set, or get extras and add more in.
Changing strings to a higher gauge will make the problem worse.
You can also get devices like a TremNoTremol-No that will set and lock the Floyd Rose in place. It removes the ability to do dive bombs with your whammy bar, but makes sure your Floyd Rose will always be dead level while retaining the ability to use the locking nut and fine tuners.