It takes some time for them to learn how to put pressure in the nipple (they have a valve and only lets milk come out when the baby presses in the middle with the tongue) it is to prevent overflow.. and also you may be able go a number up..
This is how those yellow really basic nipples were for us so we changed to the mam nipples only.
@Carly you might need to up the nipple flow. We use Phillips avent and started with flow one and we are at flow 4 now.
What flow is the nipple maybe you need a higher or lower one