1. check/change battery
2. is that particular string enjoying a lot of fret buzzing? the tuner might have a hard time reading a buzzing string
3. did you clip properly on the headstock? or try another position on headstock?
4. did you accidentally changed mode on tuner (if you tuner have different modes i.e. auto, manual)?
5. are you tuning the guitar the proper method? if you are not sure do consult a more experienced player or search guitar tuning on youtube
6. maybe the tuner dun like you