For me, this has happenned and I think it comes from not being comfortable enough yet with the situation. I can be attracted to a person, we can be making out on the dance floor, and I'm hard as can be. But for some reason, if things get awkward when it comes to play sometimes MrWilly all of a sudden doesn't work right, so I try to dwell on foreplay if I'm really into the scene until it comes back. It can be helpful to build anticipation for the sake of excitement through increased flirting and foreplay. So I think a level of comfort can be enhanced by becoming more and more comfortable by getting to know playmates more, so when everyone does get naked it's less awkward.
That's my 2 cents......
