I'm using a flexible arm to mount a monitor to my cage but I cannot get it to 'lock' into place. Vertical lock is fine but with heavier accesories the arm will slide sideways on the ball joints. The arm itself seems fine but maybe it's a faulty copy. Does anyone recognize this issue?

I assume you meant a 10 inch arm and not a 10 foot arm. A 10" arm, i assume is like a knockoff noga israeli arm.

Yes, cheaper arms will slip in one of the joints. Some arms have a kind of adjustment for the tension on the balls, but others do not. If all the above assumptions are correct, then you either have overloaded the design of the arm.. or the cheap knockoff isn't nearly as strong as it is claimed to be.

I have a redrock knockoff and the noga arm it is imitating. The noga arm is rock solid, the redrock is slippy and jerky to adjust. superficially they look like the exact same design, so something else is to play.

If in fact you have a 10 foot arm for your monitor, WOW!