Have you ever had a time when you couldn't come up with a good drawing? You try and try, but - nothing? Everyone has times like that. Many people recommend taking your mind off of your work by taking a break from drawing. They tell us to go for a walk along the beach, or go to a movie, or something along those lines. Sounds right to me.
But unfortunately, it doesn't usually work. In fact, I've met very few frustrated cartoonists wandering along the shore. Pros don't often approach creative frustration by taking a break.
A brief disclaimer before I go on: None of what I say is meant to be interpreted as an ironclad "rule." Whichever way that works for you is the best way. That said, if you are experiencing sticking points, and are looking for a way to effectively deal with them, then you might want to consider the following:
Professional artists have deadlines. Those a great motivators. But you may not have a deadline. So what, exactly, can you do to kick start your creative juices?
Start by redefining what your imagination is. It's a tool, to be used, not a sentient being, which must be allowed to wander at its own pace and in any direction. Press harder on your imagination to produce results. Turn it into just another element of drawing, like visual memory, reworking, and polishing. Imagining is nothing more than a technique to be applied to a problem.
I have immense respect for the imagination. How it works remains, to me, somewhat magical and mysterious. And yet, it seems to respond better if it's not treated as something unknowable, but something workable instead.
Many people believe that the imagination either flows or it doesn't. Were that true, you could never count on your own talent. It might be there, it might not be, depending on the flow. No one can afford to work with such a capricious muse. You need to undertake a personal journey to find inroads into your own creative thinking, and devise a familiar and workable approach to taming and manipulating your own creative thought process.
Push if it doesn't flow. If the flow is absent, don't accept this as your imagination taking a vacation. Call it back to work. Think in a new and stimulating direction to reinvigorate it. Yes, by all means, allow your imagination to wander, but continue to remain at the helm, redirecting it when it attempts to veer off course - which it will. Sprinkle in fresh thoughts, which might begin with, "What if I did this..."
Directed daydreaming produces creative results.
Just some pencil shavings for thought.
I hope that was helpful!