Number shows not the person name.

It is possible that you have not saved the number in the name of person whom you are sending message, or you might have saved that number twice in your cell phone, i.e. either in sim card or in mobile phone or both.

There is another way to check this. Try dialing the same number, if it shows the name of the person whom you have saved then its fine. If it shows the number only, then it indicates that you have have his number saved twice.

Try, you will get through this.