Brick retaining walls that have soil buildup on the other side of them are notorious for blowing the paint off through hydrostatic pressure. The water in the soil tries to migrate through the wall, and your paint seals the outside of the wall. This pressure causes the paint to delaminate and flake off.
To ensure you give the paint the best chance, I'd recommend first painting the wall with Crommelin'sAquablock. You can then proceed with using a quality exterior paint likeDulux Weathershield.
Unfortunately, depending on the amount of water that gets retained behind the wall, there's always a chance that your painted coating will fail. If it does, you might like to explore other decorative options like screening or cladding the wall with fibre-cement sheeting. - MitchellMc