The black colour is the original object, and the selected meshes are what I duplicated, and mirrored. However, the middle triangle never get mirrored. I do not know why? every object has the same pivot point (Center) what should I adjust to solve this problem?
2 ways you can mirror
1st method : If yes then delete the half. And the part which you want to mirror is loose or say separate parts so join Ctrl+L and make it one mesh and then mirror it.
or if you do not want to join and keep separate
2nd method : Just group the half you want to mirror. with selected half group press shift+D and without moving just rotate 180 on x axis. (As the 3d cursor is set)