Tracey-
I had an extremely similar situation when I first had Joey. For whatever reason, he didn't latch on to my one side the right way. On the other boob, he was perfect. When I went to the lactation consultant she watched me BF (very uncomfortable, btw). She noticed that he tucked his lower lip in on the "bad boob." That caused him to pinch the nipple and not got enough milk, hence the bobbing on and off. Watch your LO's mouth when she eats. Are both lips open around the nipple and part of the areola? They should be, and if they aren't, that's your problem. I had to use my finger to pull Joey's lower lip down as he latched on. After a few days, he did it on his own and my poor boob finally felt better.
Also, if it IS slow let down, they tell you to massage up that boob before you put your LO on. You'll know if it worked because when you squeeze, milk will come out (my DH was fascinated by this)!
I hope I helped, and I hope I don't sound like a know-it-all. That's just what worked for me.