Some of them never get up.
As far as 'pinched' nerves...sometimes there's just a lot of bruising/swelling around the obturator nerves, that can be helped with corticosteroids and NSAIDS - but sometimes they are damaged beyond repair.
And, cattle are so large, that if they're 'down' for any significant period of time (we're talking HOURS, not days/weeks), you start getting muscle damage, merely from their own weight pressing on the muscles on the 'down' side - and if you're not lifting them or at least flipping them 2-3 (or more) times a day, even if the nerve damage resolves, there may be enough muscle damage that they can't get up.
I know they're sort of a 'necessary evil', but I despise 'hip-lifts' - but have used 'em when I've had to.