It's my first year growing anything edible. I planted two tomato plats.
I started late, but fortunately, one of the plants is still producing
quite well. The tomatoes have very good flavor, but the tops never seem
to ripen and the core is green, tough, and extends to the middle of the
tomato.

Is is just the type of tomato I'm growing (unknown variety) or is there
something I'm not doing right?

Thanx for any suggestions.