I was married once. Some things that reduced the amount and the spark of great sex was daily probs that a couple experiences. Maybe you and/or your partner is stressed about work, family income, anything. It helps to communicate. I know that now coz I certainly didn't and now Im divorced
I have many married friends who are doing great in that department and its due to open communication. It helps that each person treat each other as friends coz friends rarely take things to heart, especially during a disagreement.
Good luck and good sex, my friend.