my 11 week old developed a similar problem about 3 weeks ago - after coming off one side I would burp her and try to put her back on, but she would fight it - I first thought she was full so gave up forcing her - but then last week, worried that she wasnt getting eough calories over the day I tried switching sides once she protested - she went straight on. Sometimes I then try and switch her back to the original side after 10 mins, and sometimes she takes it or doesnt. I was also told it could be a stiffness, or that I naturally hold her more comfortably one way or the other. Definately worth trying the other side tho .
Another thing to try. when you LO comes off and wont go back, try squeezing some milk out and see if it flows ok, and what thickness it is.... more often than not one or more of my ducts squirts like a fountain as if it was blocked until the pressure of me squeezing freed it.. After I have 'balanced' them all to the same flow, she seems to go back on a bit more happily... might be a red herring but I just keep experimenting.