Hello,
I have opened cow level in DIIR and I was traying to do this some time later as I saw that I can do this more than once.
Unfortunatelly it is not possible for me. Portal is not opening.
What should I do in this situation??
Thank You and have a nice day 
I have the same problem
Make sure you have the 2 ingredients to open the cow level and that you killed Baal in your CURRENT difficulty.
@Frozen If your first cowlvl was in NORMAL and you now try to open it in NIGHTMARE then it won’t work unless you killed Baal in Nightmare diff. too.
Well right now I’m finishinig hell and I’m unable to open it on nightmare.
Also I have ingredients
I was thinking of reinstaling the game but I do not know if this will help
that won’t help. you need wirt’s leg from the right difficulty and a tome of town portal and to stand in act 1 town.