An oven that's too cold will result in macarons that are wrinkled with little to no feet formation. An oven that's too hot will also hinder the ability of feet to form because the macaron will bake too quickly.
On the opposite end, an oven that's too hot may cause slightly spread macaron feet as well as possibly some browning around the edges. Older ovens that are not heating properly may cause macaron feet to spread due to uneven heat.
Oven temperature can be a cause for ruffled feet, along with not using parchment paper or a silicone mat to help evenly distribute heat.You can also try baking the macarons on two baking sheets (this helps distribute heat), or propping open the oven door with a wooden spoon to allow some of the oven heat to slowly vent.
Another common issue related to flat macaron shells with no feet 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 feet.
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.