If the report you are checking is running for a very long time - where would you put your break-point ? Run transaction SM50 - "Process overview" and find the report that is running (while it is running of course). From this transaction the debugger can be invoked: menu Administration > Program > Debugger. From here it should be quite easy to find out why the report is taking so long. An endless loop ? A while with a condition that is never met ?
