It may be because your baby was bottling your expressed breast milk while in hospital and discharged home before having "trained" to drink directly at the breast. If this is the case, you can just continue to put your baby at the breast at each feed, as drinking at the breast may be a bit more difficult initially for some babies, but this is usually only transient and once your baby has the proper position at the breast the breast-suction reflex should come back. Before your baby is able to feed completely at the breast, continue to express your own milk and give it to your baby after he/she has has gone to the breast to satisfy his/her hunger, preferably with a cup instead of a bottle as the cup does not usually interfere with drinking at the breast. You can ask if the lactation consultant from your hospital can give you advice on tools to breastfeed successfully. Your baby also probably had a fortifier added to your expressed breast-milk while in hospital, which may change the taste of the milk, but he/she will get used to the taste change and you should be able to continue breastfeeding if this is the case. Your baby also needs a bit of time to get used to his/her new environment and time schedule at home. Also make sure you have a healthy life with lots of sleep, a healthy diet and that you drink a lot to keep your milk production up to your baby's demand. If you are stressed, your baby will sense it and may not feed and/or sleep as well. You can also ask an association of parents of preterm infants or simply your friends with small babies their trick to make a successful transition to home, as it is a real challenge to leave the hospital reassuring setting with a preterm baby!
Best luck with your breastfeeding!