I have had the same issues with my husband, we have a great relationship however the sex kept becoming farther and farther apart. At first I thought it was me, but we have such a great open relationship I asked him and he said no it is not you and he has always been like this, well he was at the doctor and found out he had low testosterone level and that was causing the problem. Now it is crazy lol, so have that checked out, also some people just have a lower sex drive too.
Good luck.