What salt ? Are you dosing anything? Have you tested w/ another brand test kit? How old is the tank? Just things I would check first. Too hard to come up with a solution w/o more info.
If you really want a good answer, put all your params and anything you might be doing including any and all info on the tank and post the question on the reef chemistry forum. There are some chemistry geniuses like Randy on there. Just know that the more info you can provide, the better response you will get.
good luck,
Noah