Why the SQL statment may or may not return a result

Why the SQL statment may or may not return a result

 

  

Hi DBA's
SQL Server 7.00, I have SQL statement that contain several left joins
Problem appeared when running the same SQL statement several times – for
example 10 times, it returns results for 1 or 2 times and 0 rows for the
rest of times
Can any one suggest a reason for this?
Regards
MS Sql Server LazyDBA home page