I echo what the previous poster said. You should get in the habit of tuning your guitar each time you sit down to play and assume that it will be slightly out of tune each time. As they stated I also check my guitar while I am playing if something sounds off. I often find that after playing for some time I need to make slight adjustments while I play.

Not sure about how to troubleshoot the issues with your tuner but the previous poster sounded like they had some good ideas.