I had a little trouble with my ExStik pH100 meter over the past couple cheese efforts. I could get the meter to calibrate at 7 but it refused to calibrate at 4, and instead, dropped way below 4, as pictured here.
Prior to my most recent make, I remembered a fix-it tip to try in case the meter is misbehaving. Like a lot of electronic devices these days, this is similar to a PC...having a microcontroller inside it to take input, sample, and display results. As with computers, sometimes you need to administer a Vulcan nerve-pinch (power-cycle them). Such was the case with my meter. I removed the batteries, went and did something else for several minutes, then came back and reinstalled the batteries. I was then able to successfully calibrate the meter. Ta-da!