@elenasmommmy, yes a nipple shield is what you need

I'm using the bigger shield

have you checked for a lip or tounge tie? it can cause latching problems and can be fixed. don't forget to put something like lanolin on and maybe even a cool pack after pumping

have you tried a nipple shield? that's the only way I could get my son to latch

also make sure your pump isn't pulling to hard on them.

make sure you have the right size shield thingys