Keep trying. Astral Projection just doesn't happen the first time you try, it takes practice. I don't know for my own account, but the Monroe technique does work, and is proven so.
Don't focus so hard on the concept of actually separating from your physical body, just relax and allow yourself to feel weightless against the pull.
Use visualization if it helps.
I've done astrial projections before, using a different, and I think easier, way.
First, lay down in a quiet place. Meditate by humming or thinking or repeating a mantra over and over. Now that you are in a peaceful state, close your eyes, and ( I know this sounds weird) wiggle your toes or just focus your mind on them. Now image that a giant eraser is erasing your feet. You should start to feel a tingling sensation in your feet. Move the giant invisible eraser up your legs, and all the way to the tip of your head. You know that you did it correctally if your entire body feels light and tingly. Now that you've sepreated your astrial self from your body, try the Monro technique, and see if it works!
Be careful though, your body is weakest while preforming astrial projection. If you leave it unprotected while projecting, something could get into your body, such as a bad spirit, or a demon. Make sure to preform a protection spell before a projection, or wear a strong protection charm.
another astral projection technique that I find useful too, that wasn't mentioned is to recreate the sensation of you falling, while remaining still. This sensation is where the entire body is relaxing at a fast pace (this will take a good bit of practice to get) and once you start to feel the sensation just let it go, you won't hit the ground, you will just literally "slip" right out of you physical body.
From there you can go to the astral plane or stay in the etheric, and do what you need to do.
This has already been mentioned but it's very true, astral projection takes a lot of practice to master, for example it took me 6 months to figure out which technique worked for me and to get it to work, and another 6 months to perfect it. I'm still practicing to this day and it's now 5 years later, I'm very proficient at astral projection and can use most techniques to do so, just remember that the trick is to astral project while remaining conscious
I agree with silvervixen. that is how I first left my body. I imagined myself on a magical military cot and imagined it lifting me up higher and higher while chanting light as a feather stiff as a board until I felt I was ready. I then dissolved the cot in a shower of stardust and fell. The sensation will bre real so fight the urge to rouse yourself. What happened was that I fell asleep while falling and woke up outside my body.
