What you're doing is fine; I am not sure then why the first two jars are contaminated. If you followed the same procedure for each jar and have had the heaters on 81F you'll be alright. The heaters need to stay at 78-81F for optimal temperature, although they can go as low as 68F and as high as 90F. The jars are a few F higher than the heaters, so 81F is perfect.
As for mycelium growth, you should birth one week after you can pick up the jar and all the sides are covered with mycelium (the sides and the bottom). This gives the mycelium time to consolidate.