Im not sure how it works! But Jaspers powers are physicaly! And She can only block powers that are mentally! So he can control her emotions while others cant get to her! I hope i could have been some help!!

Alice does say why, i cant remember which book in but she does. I'll do my best to explain.

So, any vampire who has a power that effects the mind- so that would be Edward, Jane, Aro and then Kate, Zafrina etc in breaking dawn- cannot use their power on Bella. Nobody knows why that is- not even Bella. Edward descibed it in twilight as her brain running on a different frequency to his. Bella has a very private mind, and no one can touch her there.

Now Alices power does wok on Bella as she can see the outcome of a decision (not why the decision was made) thus allowing it to work outside the mind. As its outside the mind, Bella's futue can be seen.

Now with Jasper- He PHYSICALLY controls the glands in your body- the glands that release whatever chemical he wants at that moment, so he can excite or calm someone down. Many people say that your brain controls these glands and so the power shouldnt work on Bella, however, his power doesnt work by going into bella mind and "asking" it to release the hormones, it actually physically influences the glands. Since, again this power does not require jasper to get inside the mind, he can use it on bella

hope this helps x

The book actually does answer it. Its answered in Eclipse. Alice told Bella that the reason why Edward, Aro, Jane , Alec , etc. powers dont work on her is because all of there powers exist in the mind. Like Jane who makes your mind think your in pain but physically your in no pain. but Jaspers powers affects the mood or your emotions. Some kind of mental block Bella had and of course in Breaking Dawn it was manifested.