The message light on/off command is issued by one of the voicemail ports. If all ports are busy, or if there is a lot of voicemail traffic the system will "choose" to take the next message rather than turn on/off a light. This is seen as a lower priority.
Depending on voicemail usage and number of users on the system, 4 ports might not be enough. Test further before upgrading, but keep in mind it may come to that.