I have owned a pair of adult gliders (male & female) 4 months ago. I intend to breed them but unfortunately she is not pregnant yet. Can I know why? How to create a breeding environment? I have given enough proteim i guess - milk powder everyday.

I believe you should be an experienced sugar glider owner before you start to breed. There is so much to learn about behavior, health, diet. Plus learning to bond with your glider, so they trust you when they have their babies, so they feel it is a safe place for babies. I have learned alot about gliders, but know almost nothing about breeding, that is a whole new process.
I would say get to know your gliders, let them get to know you, maybe they need to feel it is a good place they are at before the female is ready.