Growing in a greenhouse

I have a greenhouse that we used to start vegetable plants in and also a couple of tomato plants per year. We haven't grown anything in there at all for 3 years. The last couple of years that we grew a tomato plant we had a problem which is apparently still with us. As with the tomato plants my marijuana plants grew to about 3', quit growing and started to yellow. I was able to save all but one plant, I planted them outside and they are doing good now. They were growing in a large bed of about 12' x 2' x 1'. I changed most of the soil this year to well rotted manure (over 5 years old), regular garden soil and some miracle grow soil. I also have an exhaust fan set to be on at 79 degrees.
I don't understand what is going on. I was hoping to extend the season or be able to have the plants mature without getting rained on. When I first put the bed in the greenhouse I was still getting tomatoes at Thanksgiving. I thought maybe it is the humidity or something is wrong with the soil.
Does anybody have any ideas? Please
Thank you