Boost the humidity!
Mushrooms love humidity. They’re made of at least 80% moisture. So you can actually wind up with smaller mushrooms if you’re growing them in a very dry area. If you’re not in a very dry place, you can just mist your bags the way it’s explained in the instructions, but if they start seeming dry or if it looks as though your rice bag or substrate is shrinking, they might need a little help. One of the best ways to alleviate this is to create a humid situation for your mushrooms that still allows them to breathe – I like to use a small plastic bin and put everything in there with the lid slightly cracked for air. A few times a day, I’ll mist the sides of the bin with water to keep the humidity levels elevated. You can get really cool little sensors called Hydromemeters on Amazon for a few dollars and they make it much easier to accurately keep tabs on your temps and humidity levels.