Fail to display crystal report in dot net application

I created my reports using CR Developer V 14.1.5.1501 and tried to display them in my dot net application using Crystal Report Viewer 2013.

If I assigned the CrystalReportViewer object a report at the design time then the report can be displayed(either reports with built in sub reports or report with import sub reports will be displayed fine)

If I used the ReportDocument object to load the report on the fly, I will get the "Failed to open connection error..." for reports with import sub reports. The other type is working just fine.(I did loop thru all the sub reports to assign the log on information and did so with the main report). I need to use the ReportDocument object so that I can export the report to PDF form and all my reports have import sub reports. Please help!!!