My dd used to have the same issue some time ago. After she was born we tried all positions to nurse and she was OK with any arrangement. But later, I think, around 4th month, I discovered how convenient it was to nurse while lying down and I ended up with a baby that associated each feed with lying down only. It was horrible! Slowly, slowly I started working on introducing back other positions and eventually she learnt to nurse anywhere and in any position. She is spirited and she is/was also very distracted nurser, I had to avoid places with things/people that were new to her because she would just pull from my breast to observe the new scenery. That means I couldn't nurse in public. I had to nurse in a car or in a fitting room.