DVD won't start

I recently borrowed NCIS Los Angeles Season 3 from our local library and discs 1-5 worked perfectly.

But Disc 6 won't even start.

I put it in the tray and 10 seconds later it gets rejected, even if I turn it 90 degrees and try again and turn it again and try again.

So I reported that to the library and they obtained another new copy of the same disc set.

EXACTLY the same thing happens.

Discs 1-5 work perfectly.

Disc 6 won't even start, it keeps getting immediately rejected.

There was no mix-up with the disc sets either, i had both sets at the same time and both number 6 discs get rejected.

Is there anything I can check or is it a manufacturing fault?

I've googled the problem but no one else seems to be reporting it, which would be strange if it was a manufacturing fault.

This is in the UK.

I tried various options in CDs and DVDs (System Preferences) but nothing changed.

Does anyone have any ideas please?

Thanks if you can help : )