Difficulty in balancing the boat can have many causes, from the obvious to the obscure. Work through this check list.
Hand heights; If the boat is tipped to your right, lower your left hand and vice versa. If this fails try dropping the shoulder.
Body: are you sitting centrally on the seat, or are you unconsciously leaning to one side, or putting your weight more on one buttock than the other? Imagine a line along the centre of the stern, from near you to the tip, and keep your head in line with it. if the boat is tipped to your right, use the right eye to line up, and vice versa.
The tap down; make sure the hands tap down at the finish at the same time, and by the same amount and speed.
Finish; make sure that the hands finish at the same time, at about the same height, with left hand nearer the stern.
If the boat is not level during the stroke, your hands are at the wrong heights, or the blades are not squared properly.