Do you have any calibration fluid(s)? Most pH units/testers require an intial and maybe an annual calibration check/adjustment. Check with your mfg, and see - then get some. It comes in bottles and single use packets and always have some on hand.
edit: I calibrated my unit using a High solution (pH 10.0) and a low solution (pH 4.0) - but se if yours requires some sort of cal, and suggests any specific solutions.