The law prescribes that the 13th month shall be paid not later than December 24 of the year (it also says that 1/2 could be paid by midyear)
Deadline for the payment of 13th month pay from Fmr Sec Franklin Drilon
Non payment could lead to labor relations problem.
This should settle the issue about follow ups of 13th month and the delays related thereto
For your guidance