Please check your T4 slips > Box 24 and 26 cannot be zeros.
Your CPP pensionable earnings are reported in Box 26 > if you have zeros in this box, you will get back the whole amount you paid in Box16.
Aso check Box 28 > you should not answer yes to exempt CPP or EI
Make sure that you have reported the proper CPP amount in Box 16 for all the T4 slips.
I hope this was helpful
