Blue oyster caps closed

Hi,
I was wondering if anyone can help I'm growing blue oyster mushrooms indoors currently. They are fruiting well but caps remain closed and not flat. Can anyone explain why they remain closed?

Welcome to Permies!

Are your mushrooms getting enough fresh air?  These organisms generate a lot of CO2 so fresh oxygen and proper humidity are important at the fruiting stage.  If you can describe your culture conditions and post a picture that might help.