ok if these are produced by the person creating the video then my guess is that they are a fitness expert, not a dvd expert.
poor quality media, plus a wishy washy burner, plus out of date firmware, plus multi-tasking while burning, plus having a machine with questionable specs to perform the burning = poor burns that may not last very long.
it's not that all DVD-Rs are junk and and will deteriorate...it's the production cost and quality that are subpar and the person producing the discs is to blame for not being knowledgeable enough to produce a quality product.
also, there's the horror of stick-on labels. if these are for sale and the person selling them wants them to look good, i really hope they're not using the printable labels that you peel and stick to the top of the dvd. those labels are nightmares and will, in and of themselves, cause poor playback especially on computer drives.