The EI contributions on line 31200 or 31217 are calculated on form T2204 or on a Schedule 13 (depending on whether you have self-employment income). You can see how the amount has been calculated by showing the relevant document in the Documents section. Any refund you get for an overpayment of EI is shown on line 45000 of the T1.
If line 31200 is showing $0, but you know you aren’t exempt from EI, you likely entered $0 in box 24 of your T4 slip. You should leave box 24 blank. Alternatively, you may have indicated you are exempt from EI by checking off box 28. You should uncheck that box.