When simmering stock you want to leave the stock uncovered while the stock is simmering.
Do not allow the stock to boil because that ruins the stock and all you need the stock to do is simmer.
You can get the stock boiling first and then after the stock begins to boil you can turn the stock down to a simmer and leave the simmering stock uncovered.
However you can partially cover the stock with a lid but leave it half open to allow the steam to escape and to prevent the stock from boiling over.
Make sure the water or liquid does not get too low when simmering the stock or it will also burn the stock and ruin it.
If the stock needs more water add some more water to it.