This is why I've had some success helping other people with their claims. I've learned the "right" way to act, the "right" way to fill out the forms, and I don't do the one thing that fucks a lot of claimants for disability benefits over: I don't do that stiff upper lip bullshit. I write a full explanation of the pain, fatigue etc caused by carrying out various actions mentioned in the forms, the implications of various medications etc. I've managed a few good results, including an "indefinite" medium care, high mobility award for an Urbanite's other half, and an "indefinite" high rate attendance allowance award for a relative. If you play the game according to how they play it, rather than merely following the so-called advice that they give you with the forms or from their helpline, success is far more likely.
Problem is, you generally have to learn this shit the hard way, from getting fucked over by trusting the DWP to be compassionate, or even "human", rather than bureaucrats.