Dear Haddadi

I agree with the suggestion of Ms. Marta

Check with the environmental factors and nutritional conditions

I would also say to check the water temperature. Also the fish age does matter. If they r too old they will not mate.  Alnogwith that keep the male and female apart for 2-3 days.Other suggestions are same as Marta.

How old are the fish?  Keeping them apart for a couple of days is a good idea. You can use a divider too or some plants.  My mating pairs some times go as long as a year or nine months of age.