In Chapter 9, "Theory", Edward explains this phenomenon: "maybe your mind doesn't work the same way the rest of theirs do. Like your thoughts are on the AM frequency and I'm only getting FM" (page 181). It is speculated that Bella's mind works differently from other people's and she doesn't send out the same brainwaves or whatever it is that Edward usually picks up on. Later on in the books, it becomes apparent that Bella has a gift of being able to shut others out of her mind and protect others as well. She is a shield of sorts whose mind is not touchable by those with certain powers (like vampires who can read minds or do various forms of mind control or state-of-mind altering).
He thought that his mind may be tuned to FM while she is AM I believe, but truthfully she has an invisible barrier that protects her from Edward's and anybodys power to read her mind. Bella had to learn how to utilize her power to save her loved ones and protect them.
Edward can't read Bella's mind because she has this invisible barrier that protects her from mind effects. After she became a vampire, Bella was able to actually sense that power and utilize it to help protect her loved ones. However, she had to train to be able to control it.
In my opinion it's because Bella was born with a potential to a power. When she was turned into a vampire Edward was still unable to read her mind, which he thought would be possible since they would be some what the same. However Bella has the capability to "shield" almost anything mentally except for physical attacks. At the end of Breaking Dawn Bella is able to move the shield from her mind and allow Edward to finally see her thoughts.
it was her destiny i guess you would call it to become a vampire. when she finially get turned into one they find out that she is a sheild. they explain it in Breaking Dawn
Bella was born with the ability to not be affected by mind effects. She has like invisible shield protecting her mind unware to anyone. When she became a vampire she was able to actually feel it's presents and control to shield others.
because hes a vampire, not a freakin phycic