problem with 'in' operator, if used with select command

problem with 'in' operator, if used with select query

sometimes it does not return all the records,
for ex.
select * from modelmaster where model in ('a','b','c')

sometimes it returns partial recordset. (most frequent with complex quries)
why?

Did you see the replies to your previous post with subject:
The 'Select' query sometime does not return all the records with .