Why can't I get accurate reading on my heart rate wrist monitor?

Possibilities. One common reason is if you have an irregular heart rhythm like atrial fibrillation. Other possibilities are technical: fresh batteries? Correct positioning? Good contact? Does it work on other people? If not, it may be defective. Has is ever worked on you? If so, what's changed? If it's never been accurate, it may not be a good unit - i'd advise an arm cuff if you decide to replace it.