What did you cover the dough with? Lots of recipes suggest a kitchen towel, and while they look pretty, the dough beneath may not have enough protection from the air.
I collect those shower caps they give you in hotel rooms, and spray the inside of it with Pam or with canola oil from my Misto. Then I slip the cap as a cover over the bowl and leave it there for the duration of the rise. You can see thru it to see how the dough is coming along, and the oil on the cap keeps the dough from sticking to it.