I can feel your frustration! It is true that acrylics usually do not lift in this way, so let's see if we can figure out why it's happening to you. I'll run through a few possible factors that might be causing your acrylics to lift. See if any of these factors ring a bell with you.
If you can change anything based on these suggestions, give it a try and monitor whether it has helped keep the dried acrylic on the board, where it belongs!
Drying time
One thing to keep in mind is that acrylic paint on board doesn't dry all the way through as quickly as acrylic paint on canvas does. When you paint on canvas, there is a flow of air reaching the paint from both the front and back of the canvas, which makes it dry faster. With a solid board, the air reaches the paint only from the front, so the inner layers take longer to dry.
So even if the acrylic paint feels dry to the touch, it may still be wet underneath.
If the first layer of clouds is relatively thick, then it might not be dried all the way through even if you use a hairdryer. Golden states that "films of 1/4 inch thickness or more will take months and even years to be completely dry." Humidity and cold temperatures can also slow down the drying time.
Even if the layer of paint is relatively thin, it might be best to let it dry for a few hours or even overnight before you attempt the next layer. If you usually rework the painting sooner than that, give it more time to dry as an experiment to see if that helps prevent the acrylics from lifting off.
Basically, if the paint has not fully dried all the way through and you are glazing over top with water and a thin layer of paint, this might be causing the acrylics underneath to lift.
Diluting the paint
Acrylic paint consists of pigment suspended in binder. The binder is what makes the paint stick to the surface - in your case, the board. If you dilute the acrylic paint too much with water, it may weaken the binder. This means that the paint may form a weak bond to the surface and lift off the next time you go over it.
To thin the paint and create a glaze without breaking down the binder, you should use an acrylic glazing liquid instead of water (that link takes you to Blick Art Materials, where I get a small commission that helps support this site). When you use an acrylic glazing liquid, you can thin the paint as much as you need to without worrying about weakening the binder.
If you want to stick with water, generally try not to dilute your paint with more than 50% water to 50% paint.
