no notification sound with subsequent message

I am having a problem wit my text messaging. I get a notification with the 1st initial new text message but no notification sound with subsequent message if i stay in the text message screen. If i leave the screen it sounds, bu tit didnt work this way before. seemed to happen after the phone upgraded the software. is there a simple way to fix this or a setting i need to do something with...i havent been able to find anything and no it is annoying that you have to look at the screen or go out to hear a notification.