Try using baked baking soda when you need to dose for alkalinity.
That'll bring the PH up.Also look and see if you can get some fresh outside air to the tank.That'll help.
My tank gets a 2 teaspoon dose every night.Thats what it takes to keep my alkalinity stable.That'll bring my alkalinity up from 9 DKH to 10 DKH,which is where I try try to keep it.