"I don't feel like practicing today!" Have you ever felt this way? What did you do about it? Did you practice? Did you practice but you didn't fully put yourself into it? What do you do when you "don't feel like it"?
When you don't feel like doing something, anything, you are feeling a symptom of something else. How did you get to that place? Did anyone criticize you too harshly? Did you take your eye off of the goal? Did you ever have a goal? How can you get back "in the zone", the place where you want to get things done?
The first thing to know is that you are not alone. Many of the dedicated and wonderful professional recording artists may not be consistently self-disciplined enough to practice every single day. Does that surprise you? Is it a shock? You better believe that most professionals do, however, prepare for recording or for performances. Those are real and achievable goals for them and it is rare to not prepare or not to care to prepare.
Everyone hits plateaus. Everyone. Truth is, we are not in a constant state of improvement. We may be improving other things about singing performance, though. If vocal technique is on a plateau, why not work on musicianship or diction or breathing or expanding your repertoire or learn a new style? If you're on a plateau in one area, you are not on a plateau in all areas. There is also performance technique, including movement, gestures, facial expressions or dancing ability. There is always something that you can do; not to neglect anything, but alternative approaches can get you out of a rut or a slump.
You eventually get to a point to where you discover that singing is 100% mental or 100% spiritual, depending upon how you personally view it. Sometimes you are not on a plateau at all but your head is in a bad place. If you hit this thick, tall, and wide concrete wall, walk to the end of it and go around it. There is no need to try to go through it. On the other side of your self-created wall is what you have been hiding from. Inspiration is there. Motivation is there. Oh my God! Your goal is there; the one you made or the one to be made. In the past people, places or things inspired you. What were they? What has motivated you in the past? It's on the other side of that wall.
Eventually you realize the wall is imaginary. It's not concrete. It's not thick, wide, or tall. It's not there at all, that wall. Don't make new ones. Life and art can be hard but only as hard as you make them.