If your oven is too low, you'll notice that the macarons won't properly rise. Not only will they be flatter than a normal macaron, but the feet will be short (under 2 millimeters), and the tops will likely be wrinkly or indented.
On the other hand, if your oven is too hot, the shells will be flat because proper feet will not form. The macarons will bake too quickly on the outside, leaving a moist interior that doesn't properly rise.
Another common issue related to flat macaron shells is over macaronaging. Macaronage is a term that refers to folding the dry macaron ingredients (almond flour and confectioners' sugar) into the meringue. This process smooths the mixture and helps the macarons bake with proper structure.
Before baking your macarons, it's important to allow the shells enough time to rest. Without proper resting time, the macarons won't rise to their full potential in the oven, typically resulting in a flat macaron.
If you live in a climate that's very humid or rainy, this could lead to problems with your macarons drying. Try running the air conditioning or a dehumidifier in your kitchen to take some of the moisture out of the air. Don't run any water in your sink to avoid adding excess moisture to the air.
Adding an overabundance of food coloring might be causing your flat macarons.Even if you're using the suggested gel coloring or powdered food coloring, too much can cause the macarons to flatten in the oven.
If you're developing your own macaron recipe, your ratio might be incorrect. If you consistently see flat macarons with no feet and a wrinkly-looking top, you might need to add more granulated sugar to your recipe.