I would recommend comparing the code you currently have with the code in the code library sample to see what's the difference causing the issue. For one, it may be that you are using an old version of the Telerik controls that has issues, especially under the current browsers. Also, in R3 2018 we upgraded the underlying Kendo scripts that power the client-side export and so things should work better in the latest release.
Last, but not least, please make sure there are no script errors and that the page markup is valid.
If neither of these helps you get the current rendered page exported, I advise that you isolate this in an MCVE (https://stackoverflow.com/help/mcve) and open a support ticket where you can send it to us so we can take a look.
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