Please test setting KeepTogether to false for all sections of the main and the sub reports. Then do the same for the SubReport item. If you are using a Table item in the sub report, set the item's KeepTogether to false as well. For more details, please check Problem: Telerik Reporting renders blank pages.
On a side note, in case sub reports contain only a Table item, please test moving the Table item in the main report, where the data can be filtered with an expression like =ReportItem.DataObject.FieldX
The suggestion is illustrated in the attached video (SWF file) which can be previewed in IE browser.
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