What’s wrong with my beetroot?

I harvested my little crop of beetroot the other day. I left about 3-4 cms of stalk and the roots intact, washed and boiled gently for about 2.5 hours. After cooling, I peeled gently with my fingers before going to slice and pickle them. However half of them are soft and perfect and half of them are like wood? Size doesn’t seem to make a difference. Some of the bigger ones are like soft butter and some the size of a golf ball are like woody stringy bullets. What’s wrong?