Painting exterior wall

[Spinney]

Hi
Have an external garden retaining wall (i.e. not part of the house). This is brick with a very thin rendering over parts of it. Part of it was painted last summer, but we ran out of paint and left the rest.

We painted the rest last autumn, but now bits of the render/cement are falling off. Some of this may have been loose to start with, but not all of it, I think. This is only happening on the bit we painted most recently, so it must be something to do with the conditions in which we painted it (we used exactly the same brand of exterior masonry paint both times).

Any idea why this happened? Is it anything to do with the paint possibly being applied when the wall was too wet? If so, the wall behind the good paint will also get wet from water soaking through from the ground above, so why hasn't that started to lose its cement?

I've got pics, but don't know how to attach them!