Why is my baby having so much trouble latching when I try to nurse her?

Many reasons. There are so many reasons that a baby may not latch properly. And there are many resources for you- ask you doc about lactation consultants (the "gold standard qualification is one who is an "ibclc") in your area and get help. We love to help mothers overcome their problems and meet their nursing goals.
Nipples shape,baby. Your pediatrician or obstetrician can tell you if your nipples have problems e.g. Flat, inverted, etc this can prevent your baby from latching on to your breast.That is your nipple is drawn to the back of the baby's mouth and your baby's gums and tongue are compressing on the dark part of your breast(areola). Also if your baby has tongue-tie, (short frenulum, that can prevent a good latch.