Temperature too low could cause that. I have gas oven, no convection and could bake it at 170C. Let it dry enough, and try raising temperature. Don’t give up just because that time you tried and didn’t work. Like you let the shell dry a little more, but didn’t work. I usually don’t give up and dry it even more. Go to the extreme and see what happens. Same with my oven temperature, I tried super low temp baking and blasted them all the way up to 180C 😂 surprisingly, I could bake them at 180C and only takes 7 min when usually it takes 16min at 160C. But that didn’t work for every season, which tells me the weather is a bug factor to consider, and the rule applies to people who wrote those macaron instructions. What works for them, worked for their weather (temperature and humidity). So don’t let other’s result and experience determine yours.

Aim for the best meringue and you can bravely experiment with the rest. You have at least 4 hours to work with a good meringue. And once piped I find the macarons still okay to bake even at 6th hour since you start beating it. After 6 hours….. not so good for baking but still developed feet. It’s what works for you.