Focusing problems are the most common eye disorder. And people may suffer from focusing problems from different factors. Generally, myopia (nearsightedness), hyperopia (farsightedness) or astigmatism and presbyopia are the most common refractive errors that cause people difficult to focus target subjects. So, it is hard to see what the problems on your eyes are. You shall take a complete eye exam so as to know what the causes. So, just talk to an eye doctor.
Are you under a heavy stress? If so, you should learn to relieve your stress and you can focus your eyes again when you are in a light mood. The phenomenon that you cannot focus your eyes sometimes can be due to eye strain. Under such circumstances, you should have a good rest and do some eye exercises. Massage can also help relieve such symptom. If you are too anxious, you can go to hospital for an overall examination and receive relevant treatment under the guidance of the experienced eye doctor.