Balancing radiators

Looking for a bit of advise: I am having a nightmare with my heating over many years. I've just had a plumber leave who has told me (yet again) something completely different than the last few I've had out to look at the system.

In short, I have never been able to get all my rads heating properly. I have tried the many youtube videos there are about balancing but I always seem to be left with one with one or two rads half hot. This latest guy told me the rads might have "sludge" in them which is stopping the bottom from heating. I am not sure about this as I can get every rad very hot if I cut flow to others etc. The other thing he mentioned was that the pump might not be strong enough. He looked at it and changed a setting from 1 to 3 (the max) and thinks that might sort it.

Another thing is that almost every one I call make excuses when I mention I need the rads balancing! Is it really that much of a nightmare!?

Anyway, can anyone suggest anything or offer some advise!?