There could be many reasons why you THINK that your manifestations aren't coming into reality.
Doubt.
A main reason is because you are doubting. Never doubt the universe, the universe always delivers. The timing that your manifestations turn into a reality is always the right timing. You are simply ordering something and waiting for it to be delivered. You already paid for it.
Remember, Its already yours - Universe.
Visualize.
You have to truly visualize your manifestation by seeing yourself in that position. Seeing yourself and visualizing that you already have what you manifest! Example would be if I am manifesting weight loss, I would be visualizing myself healthier, running and working towards my weight loss. I would visualize myself looking into the mirror and loving my body because I have lost so much weight!
Gratefulness.
Be Grateful. Thank the universe for everything, you are where you are today due to manifesting. Everything that you have right now has been a manifestation because you have visualized it, took action and affirmed it! Examples would be, Thank you for my healthy body, thank you for all of this money that I have, thank you for the food I eat, thank you for the water I have to drink, thank you for the ability to work, thank you for the ability to wake up this morning. Gratefulness goes a long way. The universe is always listening.
Writing.
When you write down your manifestations, you have to be in a peaceful state of mind. You have to put your emotions towards it. You have to visualize it. You have to hold the thought for at least 15 seconds. you have to put your mind, heart, body, and soul into each sentence you write down.