Meatballs are delicate little creatures. So the searing is what helps them hold together and you have to be careful when turning. Make sure you have enough oil in the pan so they release. If 1-2 break apart - it's ok - they'll just add flavor to the sauce. Don't crowd them when browning - that will help too! Also - using a fish spatula works really well here. You can scrape under the meatball and turn it more easily than just with tongs.