Italian macarons

Hello,

I have been having a hard time with hallow Mac’s. I’ve trouble shoot the hand mixing, the meringue is right at a medium soft peak. I’ve changed ovens to a convection oven. Once baked and cooled they look right. But when I let them sit over night covered in the walk in or out in the work space they inside deflates and the shell is hard. I work in a commercial kitchen. I stopped using a rotating oven and started using the convection oven. The oven temperature is at 300 and I bake them 18 minutes. I’ve tried 15 minutes but I feel they are underbaked ( purple color is the 15 minutes and the pink is 18 minutes). When I was using the rotating oven the temp fluctuates 25 degrees less or higher. When I used the convection oven it fluctuates 10-15 degrees higher or lower. I really don’t think it’s my mixing but I hope I can get some help. The pictures I posted are from the rotating oven.