Exclusive access for creating triggers ??

Hi everybody, I am trying to create a trigger ( for insert ) on a table, and each time I try to run the create trigger script, it comes out saying non-exclusive access. I noticed that this table was opened by atleast 5 users ( for read ), and hence could not understand why it could not lock the table. I also tried to force it to lock the table in exclusive mode after which, the server gave me an assert failure ( dumped shmem.**** and af.***** ). What am I doing wrong and what do I need to do right ??? This table is the most heavily accessed table and is accessed all night ( we have people in the graveyard shift too !! ). So I would like some suggestions other than staying up all night to see when the table is not open !! As usual, thanks for the help Sujata