There is probably nothing ongoing that is wrong with you or anyone else, you most likely just did not have a 4-way click with that couple and that is extremely common. It is VERY hard to get 4 people to all have the same level of attraction, chemistry and interest at the same time. If you have had some good 3-ways with this couple consider yourself lucky and just appreciate that and don't dwell on the 4-way encounter falling short.
If they come back with an answer to your question fine, but the chances are they won't respond with any meanfull information and if they don't just let it go. After all if you are asking "why didn't fully participate in having sex with me?" you are essentially saying that you did not enjoy the experience (which you didn't) so even though you are trying to constructively improve the situation, it will put them on the defensive and most likely come off as whining and complaining that she isn't putting out right.
The good news is if you keep getting out there and keep a good attitude it will happen periodically. 4-way clicks are hard to come by but when they do happen (and the definately do occur
they more than make up for the near misses.
You are correct in that the more you worry about something happening the more likely it is to occur. There is an old saying that says, "your focus becomes your reality." And that means that wherever you direct your energy and your concentration that will be the microenvirnment you create for yourself. If you tell yourself that everyone will ignore you and leave you out of the mix you will give off bad vibes and everyone will leave you out. Where as if you focus on being pleasant and sociable and being friendly to everyone people will want to be around you and you will reap the benifits.
Give yourself permission to fail and accept that you will not click with every but do not focus on those people but rather focus on the successes.