Hi all,
I seem to remember that DS went through the same thing at around 4.5 - 5 months - He would fuss on his bottles and eventually give in and start drinking them.
I switched to the fast flow nipple which worked for a while and then it started again. I found that switching him from the 3 hour easy to the 4 hour easy was my solution. At 3 hours he was hungry, but not hungry enough to want a full bottle. So the next morning I woke him at his usual time, gave him an extra ounce in his bottle and this took him to 4 hours when he had his next bottle.
This seemed to solve our problem. When he is teething he does start fussing again, but then it goes back to normal.
Please keep us posted on how you get on.