Difficulty urinating is also called urinary hesitancy. It can be caused by a varieties of reasons. If you just took a surgery and have difficulty urinating, it may be because a phenomena called post-operative retention. If you have difficulty urinating while some else is absent and have no problem while you are alone, it is caused by shy or bashful bladder syndrome. Some drugs (such as some nasal docongestants, trycylic antidepressants, and some incontinence drugs) can also cause difficulty urination. Besides, it can also be caused by nervous system disorders or urinary tract scarring.
Besides all the above causes, difficulty urinating in women is mainly caused by urinary tract infection. For male, the three major causes are enlarged prostate, prostatitis, and urinary tract infection. By your answering some questions, you can tell which of three causes are more likely.