It could be a booktype problem you've got now. Some standalone players are a bit reluctant to play +R media so you have to set the booktype to DVD-ROM to fool them.
DVD Decrypter can do this at burn time providong the burner supports bitsetting. In Decrypter , there's a little book symbol to the right of the speed setting. Click this & you should be able to set DVD+R DL media to DVD-ROM. I believe that Decrypter remembers this setting.
You may be able to permanently set the BTC to DVD-ROM for all +R sl & dl media but I don't know if this is possible.
Personally, I've not bothered with DL media yet as I'm happy enough with stripped out & compressed copies fitting on single layer media, plus I'm tight with my money.