Your players may simply be picky about burned media. You can try burning these Memorex dvds at a slower speed...if they are 16x try 8x speed.
Your best bet is to get better quality media. Memorex is hit or miss at the best of times. We generally recommend Verbatim as a safe choice in media.
Even with Verbatim, choose a moderate burning speed...either 8x or 12x should be good on their 16x disks.