What you are experiencing can sometimes happen if your neck has gone through some shrinkage due to environmental conditions. It is a simple process to replace the tuner.
- Remove the peg from the hole.
- Cut a small round circle out of white paper.
- Put a small drop of super glue on the tip of the tuner where you see the splines (those little ridges around the end of the tuner that goes into the hole).
- Put the small circle of white paper around the splines so that the drop of glue will hold it in place as well.
- Now place the tuner back in the hold being careful to align it as it was originally.
- Hold the tuner in place for a few seconds while the glue sets.
- You can GENTLY tap it with a RUBBER mallet if you want to set it in place.
- It will now hold. The piece of paper is there to fill in the gap that has occurred through neck shrinkage.