Several points to bear in mind that will most likely be contributing to the problem.
The A frame is an important link holding the axle in place if you just remove it then the axle will flop about uncontrolled (round and round as well as side to side) as you try to drive and more than likely rip apart the prop shaft, brake lines etc. and damage anything else in the vicinity. You could replace it with 2 links to create a 4 link back end and build it using rod ends for less restricted movement, but you have got to be getting some serious flex to reach the limit of the standard A frame
What springs are you using? heavy duty (stiffer) springs are all very nice but restrict upwards movement of the axle more than normal so you are relying far more on downwards travel to get the articulation. As you reduce the vehicle weight then the springs will also get relatively stiffer. Also is the SLU still fitted? This limits axle drop, not a problem on standard springs, but more significant after a lift.
What bushes are you using, rubber bushes get stiffer as they age and some polyurethane bushes are just plain stiff* from new. As with the heavy duty springs, stiffer bushes have there advantages (on road handling), but articulation can end up be compromised. As the vehicle gets lighter then there is also less weight to deform the bushes.
*some designs (not makes) of polyurethane bush like the ones for the panhard rod should freely rotate about one axis allowing them to rotate as the vehicle articulates without deforming. However this benefit is does not apply to all of the bushes on a land rover due to the fundamental design of some bushes, such as the bushes on the radius arm.
Fundamentally the A frame as such should not limit movement, though the bushes and the ball joint have a limit as to how much they can move. The axle end of the radius arms and chassis end of the A frame bushes are having to rotate to get the flex so in theory a polyurethane bush that can rotate on a crush tube should be freer to rotate compared to rubber bushes that have to be deformed to get any movement, however the A frame bushes (on older models at least) are angled inwards slightly which tries to push the 2 arms of the A frame together and twist as it drops. As a result the bushes also get deformed sideways and twisted as well as rotating, so fitting a polyurethane bush that is free to rotate on one axis but much stiffer on all other axis won't be as beneficial in this application as you may think.
The A frame ball joint obviously has a limit to how far it can rotate and some of the available replacement ball joints allow greater rotation than others. Not sure just what was fitted to mine (defiantly not the X-eng one) but it allowed enough articulation to require the lower ball joint for the SLU to be cut off to prevent it hitting the side of the diff, so you have to go some way for the A frame ball joint to bet the limiting factor.
Moving away from the A frame, to the trailing arms. The axle end of these have to rotate round the bolts axis so again in theory a polyurethane bush should be better, however trying it on one of my axles that is off the RRC at the moment, the standard rubber bush allows a surprising amount of rotation. Also of note with this bush is the trailing arm can fowl the bracket on the axle on extreme droop. To get round this the bracket can be ground away above the trailing arm where there is already a semi circular cut in it. You don’t need to cut out much to allow quite a lot more articulation.
Finally coming to the chassis end of the trailing arms. I find these bushes have as much effect on articulation as any other part of the rear suspension. They have a limited arc of movement so if you haven't already got them then cranked trailing arms make better use of the available arc than standard ones after a suspension lift. This bush can also be replaced with a rod end to allow free movement.
