What software are you using to burn those Dual Layers ?
On june 22nd, 2005, the latest version of Nero is v220.127.116.11
If I recall, in the few last releases their was several fixes for Dual Layer Support.
Mainly for DVD-R DL.
Seeme v18.104.22.168 and above will support DVD+R DL but I always say latest should have the less problems.
Maybe you need to get the latest version of Nero, or whatever software you are using !
My 2 cents...