I have exactly the same problem with one of my hens and it happens about every 3 weeks now. I have done all the vitamins but found the one thing that makes a difference is giving her tuna.
I give her a teaspoon of tuna in springwater ( not in brine or oil) and within a day or so she's back on her feet again. So for me it look like a protein deficiency.
I have a thread on this somewhere but as I'm on my phone and currently cleaning out the coop, I can't find it right now. Her name is Nella if you do a search in my posts.
I think the reason why mine does it is because she isn't eating properly and she is very old, so once she goes back into the run with the others, I don't think she gets enough food and it it manifests itself as a weakness which shows first in her legs.
I keep her in a cage indoors for about 3 days, make sure she eats well, along with the tuna, and I also gently massage her legs a couple of times a day.
Good luck. Hope this is useful.