IS YOUR SQUARE SQUARE AND YOUR LEVEL LEVEL?
To test if your level is still giving you an accurately reading place it against a wall and draw a line along the top edge. Turn the level end for end and place it against the line you have drawn. If the reading matches the line on the wall the tool is accurate, if not it may need adjusting or replacement. Testing a square uses the same principle, place it against the edge of a straight board and draw a line then flip it over and check if the blade matches the line.