GRRM has said in interview after interview that Bran is his most difficult point of view character write:

The most difficult viewpoint character to write is Bran and he has always been the hardest. He is the youngest viewpoint character; which you know is difficult in and of itself because when you're writing a character that young you can't simply write what's going on. You have to filter everything through "Ok he's 8 years old what does he understand" he's seeing this scene does he actually understand what's what's happening in this scene. Like when he comes on Jaime and Cersei, well they don't have any clothes on and they're wrestling you know because an eight-year-old has a different way of perceiving these things. So you always have that issue and his language has to be carefully considered...

[Bran's] chapters I have to put together very, very, very, very carefully and they tend to take me longer to write than the chapters of any other character. Which was not to say I don't love the character and some of the stuff he's getting in is, is great, I really enjoyed writing some of his chapters and Dance with Dragons in particular, the stuff when he when he gets far north and that was the green seer and so forth, in the cave. [Source]

“And I had a very hard time…writing Bran. Because Bran, of all the characters, was the one who was most involved in magic. And I think magic in fantasy…has to be handled with a great deal of care, or they can overwhelm the story. So I rewrote some of those Bran chapters over and over again.” [Source]

Since ASOS, when Bill Clinton was president, three Bran chapters have been published. A fourth unfinished chapter we know of is set for TWOW.

GRRM Tries To Fix Bran

GRRM's difficulties with Bran (age, magic) are not new. They've plagued him for two and a half decades. Originally, the books were supposed to take place over several years and characters would grow up organically. This didn't work out. But GRRM had a solution; after ASOS a time jump would age Bran up by five or six years, meaning he could be written virtually as an adult. An interview from 2001:

Thus far I'd say the hardest character has definitely been Bran, on two counts. Number one, he is the youngest of the major viewpoint characters, and kids are difficult to write about. I think the younger they are, the more difficult. Also, he is the character most deeply involved in magic, and the handling of magic and sorcery and the whole supernatural aspect of the books is something I'm trying to be very careful with. So I have to watch that fairly sharply. All of which makes Bran's chapters tricky to write. It should be easier in the next book, I would think, with the five-year break. Then I'll have a 14-year-old, and in terms of the Seven Kingdoms, that's almost an adult. [Source]

Even after the time jump was scrapped it was still GRRM's intention to have the next book, AFFC, take place over five years. But by early 2005 this hadn't materialised in writing and GRRM still hadn't written any new Bran chapters. GRRM considered another approach for Bran (and Arya):

[GRRM] wants the 5 year gap because he wants the kids to grow up, but some of the adults have too much going on for that to work. He now thinks he needs to somehow put the characters on different time-lines, so that those who are kids can grow up, and he can still detail the adults activity. He said he has an idea how to do it, but since he felt trapped the last time he said what he was going to do, he wouldn't talk about the specifics. -SSM. February 2005

GRRM was coy with details about his idea of separating the passage of time by PoV but evidently nothing came of it. By May that year however the idea of AFFC covering five years was finally buried, yet GRRM still hoped to scrape out even a six-month time skip in the books for Bran and Arya somewhere:

He said the five-year hiatus is as dead as his plan to finish the series in a trilogy. While he would like to skip ahead to age the children (esp. Bran and Arya), he feels the back-story is too interesting and important. He needs to focus on their development. While he would like to find a time to skip ahead 6 months in the story, it's unlikely. -SSM, May 2005

TL, DR:

Bran's PoV is very tricky for GRRM to write because he's so young. GRRM didn't intend to have Bran as a child for the entire series. He tried to age Bran up in the early 2000s but failed. We know from interviews that Bran is still a major issue in writing TWOW.