Either your DVD player can't read the brand of DVD-Rs you used (it happens often with cheap DVD players), or you created DVDs for PAL instead of NTSC (or the other way around).
...or if it won't read a DVD-R, try burning it to a DVD+R (or vice-versa). That happened to me when I did a video of my nephew's wedding and sent him the final DVD. Wouldn't play. Tried burning to a +R and it played fine.
DVDA produces a dvd that is 'closed' and ready for playback on a standalone dvd player. Some older dvd players will play commercial dvds just fine, while having issues playing home burned dvds. There's not much you can do, it's just your dvd that is aging. Buy a new one. They are really cheap now.