flummoxed

I am unable to fathom a reason why the following select statement:

select * from my_table where my_id > i_last_id and my_id <= i_max_id

has returned an incomplete batch of records. In at least two cases of running the more complicated version of this statement I get a result set where the sequence of id's looks something like this:

1
2
3
4
9
10
11
12

So what happened to records with id value = 5,6,7,8? These records are present in the my_table and yet they were not present in the output of the select statement.

Flummoxed.