Welcome back, Nick.
I was always taught that anything that might cause recirculation reduced rotor efficiency, but whether a wall can do this is open to debate.
Just for fun, on a similar subject, when you have charged your little toy and solved this problem, if it's an indoor helicopter, please next try hovering it just under a flat room ceiling (I mean within an inch or so) and let us know the result!
I've tried this many times with a miniature helicopter I used to have, with repeatable results. It was infra-red controlled and had constant pitch blades, with rpm controlling rotor power. Whenever it got too close to a ceiling, the damn thing seemed to get suddenly drawn upwards then become almost "stuck" to it. I assume this is because of greatly reduced inflow giving a much more efficient rotor, similar to ground effect but much more pronounced. Having reduced rotor power almost to nothing to get it to drop away, it was difficult to stop it falling so fast that it hit the floor. Almost as if it instantly went into a vortex ring condition.