When you force yourself to work on a research problem, until you find the solution, you might be putting too much pressure on yourself and actually hurting your creative process.
Instead, you should try to take a few days of distance from your problem and let your inner gears do their work.
You can take a break by working on a different problem, or by spending time preparing teaching duties, but you could also simply take a weekend off.
Are you surprised to read that continuous deep work might hurt your creative process?
To understand why you need some time off, it is necessary to have a closer look at the essential components of creative acts. Graham Wallas  analyzed reports of scientists like Poincare to understand the components that are found in all creative acts. He identified four components:
This step involves a long period of intense conscious work, without success. In this stage, potentially useful ideas are considered. Others might identify this stage as the point where you hit a blockage.
The problem is put aside and not thought about or worked at consciously. Within the unconscious, potentially useful ideas from the preparation stage are combined in new and unexpected ways.
If the incubation is successful, a sudden illumination occurs: the researcher experiences a sudden insight into the solution. This event can be considered as an unexpected leap forwards, but is simply the outcome of your brain doing some background work while you are not fully focused on it.
In this stage, true creative work is done as the researcher manages to cut through his/her blockage.
The illumination stage produces an insight that needs further work. In the verification stage, the new insight is tried and tested. Wallas, G., 1926, The art of thought, Harcourt Brace, New York.