Worms would explain most all the weakness/problems.... they can kill them if the worm load is very heavy... So, yes, it was not what you did or didn't do, except for them not all getting wormed, and if you can't catch them, then that is understandable. If they go to the auction, whoever buys them, should as matter of course worm them as a precaution when they take them home. We worm all calves and/or cows that we bring in as matter of course.. and in fact, had some cows looking thin and rough, and had fecal samples done and had some heavy worm loads in the cows..... these are our own cows..not recently purchased..... we wormed them and they are coming back around... So, it happens to everyone..
Plus, since they had been so badly neglected before you took them on, they probably just kept getting reinfected with worms by being on the same ground and there is nothing you can do to prevent that except to rotate them off to somewhere else for a long period of time.
Also, little bits of plastic can cause problems in their gut tract... we are diligent about not letting any of the plastic baling strings or net wrap around the big round bales being left in the hay as it can mess up the cows inside and actually cause an obstruction... In your case, it just wasn't helping them in their stomachs and small pieces are so easy for them to just pick up and eat along with the hay.
Best of luck with selling them, and them having a chance for a better life than the previous owners before you, gave them. I am sure you saved some of them from death by taking care of them as best you could.