mysql_insert_id()

I'm having intermittent problems with this function. Most of the time, it returns the proper value. However, sometimes it doesn't return anything at all even when the insert query executes without error and I can see the new data in the database. I try to get the last insert id both by supplying the link-resource and not but still get the same results. When I echo out the link-resource (what mysql() returns), all I'm gettins is a numerical one (1). What gives? Chris