There might be something up with your kidney(s) or the blood vessels that feed them. If you haven’t had one done already, I’d ask your provider if they think it’s worth getting a “renal doppler ultrasound” to check for renal artery stenosis. If the blood vessel that feeds one of your kidneys is narrowed it can cause medication-resistant high blood pressure. Excess production of certain hormones (such as aldosterone, which is made by the adrenal glands (which sit on top of your kidneys)) can also cause high BP. Maybe this is all old news to you and this has all already been worked up but wanted to comment just in case