SSRS report Issue based on Controller class extends from SrsPrintMgmtFormLetterController

Dear all

I need your help please , I have a strange issue with My SSRS report

I created a new Report using FormLetter , I want the same logic than CustAccounstStatement Report I mean
Generate mulitple reports in PDF format not one Pdf contain all the data ,

So I implemted my Controller Class extends form SrsPrintMgmtFormLetterController
my ClassContrat extend from SrsPrintMgmtContract with SysOperationContractProcessingAttribute(classStr(myClassUIBuilder),SysOperationDataContractProcessingMode::CreateUIBuilderForRootContractOnly)
the issue is when I used Screen mode my report are generated correctly exemple 5 report but if I choose File (pdf) The system generate just 3 files and it is Random not always the same Files but just 3 not all the reports

I don't know if I missed something