Couples can overcome physical challenges. In the beginning, it's tough to know what your partner enjoys. So, being "off rhythm," touching the wrong spots, trying the wrong , or being a poor kisser can be fixed. Physically, you can coach each other on your likes and dislikes. After a while, you'll know what to do, and how long to do it. With work, communication, and honesty, bad sex based on physical issues can be improved.
But if it's a mental disconnect that's making sex unsatisfying, it's tougher to fix. For a good mental sexual connection, there needs to be some lust involved. In a long-term relationship, lust is rooted in the chemistry you share with your partner. It has to occur naturally. It can't be taught or intensified through conversation. Just like any other activity, sex won't be amazing every time. You're both have off days. So, don't jump to conclusions after a few bad encounters.
Finally, there is a spiritual connection that can happen during sex. This type of connection, or lack or connection, is tough to change or improve. There are many reasons that one may not be satisfied spiritually during sex. It all has to do with that person's core beliefs. People who have monogamous sex when they secretly long for multiple partners will not be spiritually satisfied. Conversely, people who are having casual sex, when they actually want a long term relationship, will feel empty. Satisfying your spiritual side sexually requires knowing what you want and working to fulfill that desire.
In my opinion, only one type of sexual "issue" can be fixed. If you're mentally or spiritually unsatisfied, you might have to think about whether you're with the right person.