Egg Bound- ongoing problems

Update on my egg bound hen - she since the last problems she has been living outside as normal with no after effects. UNTIL....
I found her last week lying down with an egg half out and yet again with paralysed legs. The egg was small but normal, she had showed no signs of discomfort or slowing down, no temperature, eating normally. I don't know whether she has been laying since the last but believe she has. We have kept her indoors again and she is now walking again, albeit stiffly. I suspect she has a recurring problem with a trapped nerve but as long as she is cheerful and not suffering I shall carry on looking after her.
The diet our girls get is good quality pellets, mixed corn for a treat, greens and insects from the garden and oyster shell and grit is available. Our vet is as puzzled as we are!