I can't record anything over the SP record mode on my DVD recorder! Can anyone help?

I seem to have a fault with my product DVD Recorder RDR-GX120. If I record anything over the SP record mode setting (anything more than 2 hours) it wont record to disc. In particular it wont finalise, and once you have attempted to do so the disc becomes obsolete and un-recognisable. I have also tried various DVD's but no luck, yet they seem to work when recording on SP (2hours).