My sourdough keeps going flat and ends up dense!

This is a problem I've had with sourdough every time I go to make it. I'm using a sourdough starter split from one my dad started. I am positive that the starter is still active (Bubbles appear when I feed it). The starter was started in 1989 if it's relevant - but like I said, it still produces gasses when I feed it, bubbles up when I open it, and so on.

I've followed a few different recipes, both with and without extra yeast added. Every time I end up with a dense, flat-ish loaf. The loaf definitely rises - just not nearly as much as I expect. I've tried things other than loaves too, and they all end up equivalently dense. I've used AP flour, and my dad says he's always used bread flour, and we both have the same results.

Most recently I used this recipe, and at each step my dough looked very similar to the photos on that page. At the end I decided to keep the batch as one loaf and make a sandwich-sized loaf in a bread pan. The dough visibly rose during the first rise, but once I put it in to the bread pan for a two hour second rise I didn't observe much of a change. The dough again rose a little bit during baking, but I still ended up with a loaf only about an inch tall.

Unfortunately I don't have a crumb pic. But basically, uniform texture with no large holes.

What might be going wrong here? I know getting a lighter, larger loaf is achievable. I have had sourdough even bought from the store which is much lighter. I just have no idea where I'm going wrong.