Read your contract.
First of all it is not a 13th month. It's an "Annual Discretionary Bonus", which is subject to the Company making "a satisfactory operating profit".
Those of us who have been here long enough remember when the based salaries were raised 8.3% to include the equivalent of a 13th month. It was included in their monthly salary. However those of us remaining in HKG get our Annual Discretionary Bonus only when CX makes a satisfactory operating profit. I think it has not been paid 3 times while I have been here.
The Company practice is to inform staff as early as possible, if the ADB is not going to be paid.