If you look at things you can have at will, like maybe a banana, take a good look at how you view that and your belief system for getting the banana. It's a small thing but the process is the same.
Sometimes having patience while it materializes is also a benefit to you. Sometimes what you want goes against an underlying belief system you may or may not be aware of.
Occasionally the time is not right for you, but things will come into play that will make it ok to have at some point.
It is a skill you develop if worked on regularly. There are several books that deal with it like Unlimited Futures, The11th Element, Anything Fast (e-book).
The Abundance For Life Course also teaches this. This is a pretty start.