Problems with balance are signs of thiamine deficiency in cats, which is caused by an unbalanced diet. There is a nice symmetry in that statement. Your cat might fall over or in trying to stay upright will crouch low to the ground while walking. Or, for example, your cat might lean against a wall for support. Seizures frequently accompany thiamine deficiency. If the cat is lifted up his or her head might drop to the chest.
Typical unbalanced diets causing thiamine deficiency are raw fish diets (in excess) that destroys thiamine which is vitamin B1.
The cure is to reverse the cause of the problem, namely to revert to a balanced diet and to administer injections for thiamine if recommended by your veterinarian.