Non-compliance. Uncontrolled hypertension usually occurs to indiviudals who are not compliant with their medications and a low salt diet. However there a few causes that are rare like renal artery stenosis (narrowing of your renal artieries) or hyperaldosteronism (excess aldosterone release that causes uncontrolled htn) that can also occur.
Yes. Much of the time, there is no identifiable reason for high blood pressure. However, certain diseases such as cushing's disease, pheochromocytoma (a type of adrenal gland tumor), some medications (including over-the-counter decongestants), narrowing of the arteries to the kidneys, kidney disease, and obstructive sleep apnea among others can cause high blood pressure that is difficult to control.
Yes. High blood pressure can virtually always be controlled with a combination of medicine and lifestyle. When your doctor and you find that you are unable to control your blood pressure after a reasonable trial period (6 months or so), referral to a hypertension expert is appropriate. There are a large number of tests that can be performed, and having someone well versed in the evaluation is best.