Maybe its time to spicen up the sex alittle. Sometimes when couples have sex they develop a routine of the same positions over and over again. Be alittle more spontaneous and try something new. That is what you probably need to fire up the rocket again. lol.
As well, if your stressed out, that could also be an issue. If your worrying about something during sex, it will distract your mind and swing you out of the mood. So see if stress is a problem.