Great question and surely there's more than one answer.  Of the various reasons that come to mind, the most prominent reasons that I suggest you feel your swing better on certain days relate to your energy level, tempo, grip pressure, and presence.  Breaking these down respectively, your energy level changes daily based on your thoughts, foods, drinks, and exercise.  Your tempo tends to match up with your energy level.  Grip pressure impacts the feel of the weighting of the club which can directly impact how your swing feels.  And lastly, if you are focused on something pressing such as running behind in your warm up before going to the tee, or personal matters such as an important phone call you need to return, you're out of focus, not present, and distracted from imagery skills, which impact the feeling of your swing.  Hence, the first of three recommendations is to develop a daily routine that reinforces quality sleep, food, drink, and exercise.  Secondly, use a pre-shot routine that includes the step of checking in with grip pressure.  Lastly, learn as much as you can about focus and imagery skills.  Develop a schedule/system that has you working those two mental skills.  Thanks for your question!  Now here's one for you... How do your thoughts impact the way that you feel?  Take care.