no sim card for second time

my iphone 5 started showing no sim card and best buy replaced the phone with a new one. once i got home and connected the phone for updates and contacts the phone started showing again no sim card. best buy is now sending out for another phone and told me that my software might be corrupted on my imac but my parents and sister have no problems connecting there phones to the imac. is it possible best buy is telling me some truth or they gave me another bad phone

any suggestions