rendering, have read the search!

Been trying to render some raised garden bed walls.
I have changed nothing from start to finish but i have 2 walls that came out perfect, and two that just keep falling off.
I can plaster (internally) and so have a good basis of skills but this is just too weird.
When applying my scratch coat big areas will slump and drop off?
I am rendering over brickwork.
If this was plaster id stick some pva on first but everyone seems dead against that?
Any clues?

my mistake, thanks

whats on the bricks? i always give a good fungicidal wash scrubbing with a wire brush, try that

maybe the bricks are damp so have no suction.