D.M.
My four month old son also does this at times. He usually does this in the evening/night when he is really tired so I warm a bootle of expressed milk and let him sleep. At his next feeding he is fine. It could be that it is at night and you have worked all day and are stressed and tired. As long as she is getting your breastmilk that is all that matters. My first son (now 4) never latched on. He was born 4 weeks premature and did not know how to suck. We had so many problems that I just pumped EVERY bottle for him for 10 months (my milk dried up by then). He had the benefits of breastmilk and I had the freedom to leave him with dad or grandma to go shopping or run errands.