There are any number of reasons that ISO don't work, bad ISO, wrong format for the usage (IE UDF 2.60 when it should be 1.02 or ISO9660, Mode 1 or mode 2 etc...).
Could be the laser is getting weak in the burner (especially if its 12 months old or older) or getting weak in the Sat Nav unit.
Also discs are not 100% compatible with all devices, and the same with burn speeds. Some older dvd players like films burned at 4 speed, some work with 8 speed, and some will work at 16 or higher.
I've been burning discs ever since I bought my first scsi drive (2 speed CD writer which cost over £600 with scsi card) and since DVD-R came out in 1997 there have been many different brand and dye combinations and I think I've tried them all.
I never get any discs based on brand name, I always buy them based on dye.
I found that the make of the burner makes less of a difference than the dye on the disc.
The ONLY disc I have ever had 100% success with are ones made using the Tayo Youden dye.
I have always liked and recommended Pioneer burners, and use one in my pc now, as well as an LG Blu-ray burner.
But I write around 800 disks a month using my 2 duplicators which have 8 Samsung burners each, and don't get any problems. I'm using discs with MCC dye at the moment (Verbatim) @ 9p each.
I've done a few sat-nav disks with no problems.