To be honest I think the issue is pretty simple, the Poch 4231 struggles against 3 man defences. It's harder for us to pressure the opposition because they have easier out balls, will be forced less in to mistakes and our striker is isolated, unable to bring in the 3 in behind.
With two strikers on, we can impose ourselves more. The strikers compliment each other and we give their defence more pressure when closing down, meaning our game plan can be put into effect.
I think this happens every time we play a 3 man defence. I hope we either anticipate the teams that will do this or work to adapt quickly in games when we come up against it.