Tough means undercooked, and you have room
Extending the simmer time meaningfully past the recipe's stated duration -- including delaying the potatoes to let the meat go longer first -- improves tenderness.
Chuck is full of connective tissue, and connective tissue is the point — it is what turns into the gelatin that makes a stew feel like a stew. But it converts slowly, and it converts on its own schedule rather than the recipe's. A piece of beef that is chewy at two hours is chewy because the conversion is not finished, and the only thing that finishes it is more time at a low temperature.
Cooking well past the recipe's stated time -- a substantially longer simmer before adding potatoes, an extra 30 minutes, simmering longer than called for, or baking 5.5 hours -- consistently landed in the good-to-excellent tenderness range (5-9/10), with the single lowest score in this group (5/10) still coming from the shortest of these extensions.
What to do: another 45 to 60 minutes, covered, at a bare simmer. If the potatoes are already in and threatening to disintegrate, fish them out and put them back at the end — two cooks did exactly that and rated the result 8 and 9.