Why are my notifications not coming through together?

Just recently my text message notifications aren't coming through on my Blackberry Classic. Sometimes the audible notification comes through, without the LED lighting up. If I hear it, I will reply to the message, then about 5 minutes later the LED will be flashing as if I have a new message but there's no new messages. The LED is delayed for some reason. I haven't installed anything recently so I don't know what the problem could be. Has anyone else had this issue.