Wow, I bet it cooks in there. Go sit in the shed with the door closed between 2:00 and 4:00 tomorrow after noon. I think you will find your problem. You might have trouble breathing also. VV
If that plant is being grown outside you need to make sure it is getting direct sun light. Transplant it into a 3+ gallon container using a high quality organic soil... mix the soil with 20% perlite.. it will recover 10% .. especially if you water it with pH 6.8 water.. check this video.