An asterisk (*) means that there was no response from that host/router. This is known as a timeout, or a lost packet.
A N/A means that there wasn't *supposed* to be a response from that host/router. This happens if another host or router responds for that hop (ie: oscillating route), or if you've stopped tracing for a time, and then resume.
In an alert, the most likely cause of this (N/A) is because of a route change - where a host/router wasn't in the route for one or more samples - and another host responded in it's place. For that situation, a N/A is shown.
