Make sure you are clear what you are asking your tuner to do. The tuner does not automatically “know” what notes your guitar should be tuned to. What it does “know” is how close the tone of any one of your strings is to the actual pure tone you are aiming for.
For instance, if you are trying to tune the note A, the tuner will tell you whether your attempt at an A is a little too high, a little too low, or right on the money.
If you feed the tuner an odd note to begin with, like a D# for instance, the tuner will tell you whether your D# is a little too high, a little too low, or right on target.
Every tuner has some way of indicating what note it thinks you are aiming for, usually a visual representation on the L.E.D. screen. If you are tuning your guitar in the standard manner, make sure you are aiming for the following notes, low-to-high: E, A, D, G, B, E.
Sometimes the electronic tuner gives weird answers when the battery is low. If you’re certain you have the notes right and you are still getting weird answers, try replacing the battery.
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Guitar Life 
If you find it hard to get a stable reading on the tuner (ie the note sends the LED screen flashing in all directions) it is because the tuner is picking up overtones from the other open strings. For example as you hit one string, other open strings will also sound in sympathy and the tuner doesn’t know which tone you’re after. This is even more apparent when using open tunings.
Solution: Simply use either your left or right hand to damp the remaining 5 strings and the tuner will do its job without the wide fluctuations.
possibly a low battery. mine does the same thing when it gets low.