What can cause a guy to not get an erection even though he is really in the mood for sex?

I have had this problem many times before. Although you are very much "in the mood" you 1. may not be sexually attracted to your partner2. may be stressed or thinking too much

Try not to think about anything at all during foreplay. This creates a pressure on you that you HAVE to please your partner. That's exactly how I felt before. Be sure to clear your mind of any problems and or stress you may be going through.