It may because you did not surpass the 3% net income threshold.  In order for you to receive a credit for medical expenses, the amount of medical expenses you claim must surpass the 3% multiplied by your net income.  The credit is then calculated by (amount of medical expenses claimed less 3% of your income on line 236).  

For more information on the calculation of medical expenses, I encourage you to visit this link :

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/330-331/menu-eng.html#...