Burned (as opposed to replicated) DVDs can run in to some compatibility problems with various players. When I do a burn I test with two consumer DVD players. Older players and some brands are prone to problems with burned DVDs and some may not like certain brands of media. Some don’t like high data rate peaks either.
I use Verbatim Hub Printable. I burn at 4x. I keep the peaks down to 6.5 (VERY CONSERVATIVE). Using AC3 audio provides more room to increase the video data rate. Actual data rate depends on how much you’re putting on that disc but limiting the peaks is key.
Even with all that some older players just don’t like DVD-R or DVD+R. As I said I test on two different consumer players so if it’s working on those the issue may very well be their player.
I’ve seen compatiblily rates reported at about 95%-98% for burned DVDs. Replicated is at 100%.
Don’t forget discs themselves can get dirty (dust, fingerprints) and dirt can certainly get on the playback laser. In other words I’d tell the client to try on another player (I recommend the 2 brands I test with – Sony and Samsung) before they blame your discs/burns.