Hi Sam,
Since your temps and humidity sound good, I would look to see if they are getting enough air and if they are getting right amount of light.
I would suggest adding holes for air exchange. As for light, most mushrooms need around 12 hours and prefer indirect light (*check the needs of the type you are growing). From what I understand pinning/fruiting is increased by the presence of light in the blue range so if you have the means you could buy strip
LED lighting online (if you arent afraid of cutting and cliping strip lighting together you can buy reels for very cheap on amazon) within those ranges.
The last thing thing I can think of now that might be inhibiting fruiting could be how the substrate is being watered. Its important that when misting the water is sprayed on the walls of the container and not the actual mycelium. I've read if water is sprayed on the mycelium it can cause pins to abort.