Unfortunately, there are so many reasons as to why your macarons are hollow. Oven too hot or too cold, under mixing and over mixing the batter, letting them rest too long, even the weather can impact it. But I wouldn't focus too much on hollow shells, some macaron experts say holes in the shell improve the overall texture. I do recommend getting an oven thermometer though! My oven is 5 degrees colder than it should be, and 5 degrees can make a huge difference when it comes to baking macarons.