First and foremost, check the blade of your miter saw. When the blade is too old or dull, this can lead your miter saw to make inaccurate cuts. Changing the blade and making sure that the nuts are well-adjusted may fix the problem.

If the issue doesn't involve the blade, please check fence and table: You'd need to check how flat the table and fence are. You can use a straight edge on each fence separately and check for gaps by using feeler gauges. You'd do the same with the table.

Adjust miter: 

1. Unplug the miter saw.
2. Use a square and hold it against the fence.
3. Lower the blade down staring by the fence side and slide the blade over until it slightly touches the blade (teeth of the blade if the square is long, or blade itself if the square is a small one). 
4. Once your square is properly positioned, check if the blade is touching the square in both close and away from the fence. If there's a gap, move forward to step number 5.
5. We would need to adjust the detents:
   • Loosen all the screws that are holding the detent in place (As recommendation, leave two of them slightly tighter than the other. That way, we can make adjustments with the detent without moving around so easily).
   • Make sure to check that the saw is still locked in to the zero degree detent.
   • Move the square over tight to the blade, but do not put too much pressure or it could distort the blade from its original position.
   • Tap the miter handle either to the left or right to align the blade exactly with the square. By doing this, the detent will adjust automatically to the new position as the screws are loose. 
   • Tighten back down the screws and follow step number 4 again to confirm the full travel of the blade.