It definitely happens and it's probably a pretty common reality. Even if I do enjoy drawing and making stories, but it doesn't mean all the work will be enjoyable. In that case you got to think of your end goal and power through it. Sometimes, it happens because constantly working on the same project isn't always as simulating as doing something new. In that case, do something different for fun. Other times, self doubt and crippling artist insecurities happen. Is it any good? Does anyone actually like it? etc. In that case, remember that your job is simply doing the best you can right now.
So conclusion, some tips on what helps me:
1)
Set a certain amount of hours (ex. 3 hours) to work on your comic each day. If you feel like drawing more one day, great, but if not, you still have x amount of hours done. Plus, you won't feel the weight of doing x amount of work by the update time when you can focus on a time goal each day. Does that make sense?
2) If you feel like drawing something else, go for it. As long as you remember to go back to your comic, it's good to stretch your wings.
3)Remember why you decided to make your comic in the first place. Why is it important that you make this comic? Even if it seems only important to yourself, that's just as a valid reason as anything else (like world domination lol)
On a personal note, tip 3 is the one I have the most trouble with since the content of my comic, Halfsoul can be quite dark and depressing at times. So instead of worrying whether or not people are enjoying the comic or not, remembering that I'm writing this story because I want to think (and others to think) about what it means to lose a part of yourself and what recovery can mean, helps.