I wear a flyye plate carrier, with hydration pack on the back, mag holders on the front etc, but the problem is that it tends to slip backwards, and I have to keep pulling it down my front again.
I find this a lot with any plate carrier that doesn't have a particularly substantial cummerbund to keep the front and back in sync. Problem usually stems from the fact that RS mags would weigh down the front whereas mags for AEGs are usually pretty light, and the water on your back obviously weighs the same no matter whether you're 'softing or firing real rounds.
So yes, as above really you need to counter that weight somehow by putting more up front. Either that or try a smaller hydro solution like a bottle or 1L (or less) bladder in a pouch on your side.