Welcome to CD Freaks, Chatboy
Just to rule out a faulty disc or an issue with a particular brand, I would recommend backing up the disc's content to your PC and then try the discs one by one to see if this whether only a certain brand of discs this occurs on or if there is one disc that the recorder keeps falsely recognising as a blank. So far I have not heard of this issue before and have not experienced this with my LVW-1105HC+ or LVW-5045 recorders.
If this is only a recent issue, in that the recorder never done this for several months and now frequently erases DVD+RW's, I would recommend running a cleaning disc just to make sure that it is not a dusty laser causing it to mistakenly recognise written discs as blank. Finally, if you have not updated the firmware recently, I would recommend carrying out this just in case it is a known bug.