Problems Burning CD's

Can you clarify "problems with them being read"? Do they not play at all or do they play but skip? Did you recently change the brand of blank discs you use? Different brands of discs will give you different looking burn patterns, and different quality burns.
Different burners like different blank media based on the burner's firmware, and the quality of the burn will vary with the media being used. Media even vary from batch to batch, the same name on the case doesn't mean that the same company actually made the disc inside as in a previous batch by the same disc vendor.
Add to that the fact that Apple rarely updates their firmware and you have a common problem with Superdrives - discs that don't play well with other cd players.
But I have had drives that slowly began producing bad burns on good discs, until they eventually couldn't burn anything useful. So you could have a dying burner or one that you just need to try different media with. Check also to see if there is a firmware update for your drive. Also, get a lens cleaning disc and use it on the drives, that will sometimes do the trick. My guess though is that since it is both drives you have a bad batch of discs. Cheap, no name discs are to be avoided as there is a good chance your drive doesn't have firmware written for them, and will have a very hard time producing a good burn.

Yes, you should be able to see the burned area, but again this varies between brands of media. The fact that it has varied within the same batch of media is strange, but I did have a similar problem once with a batch of Maxell, where the first 6 or 8 discs burned badly on a variety of burners, and then the rest all were perfect. I went crazy for 2 days thinking the burners or computer were bad and it was just those few discs. So I still think it's the discs, maybe just poor quality control on that batch. Can you try a different batch or brand to check?
Skipping usually is a sign of many errors produced during the burn, which usually means bad media, bad firmware or both, but sometimes lens cleaning does help. Do you have access to a PC? There are many programs for the PC that can test burn quality, and it's likely I think that there are uncorrectable errors at the spots where your discs skip. Car players are notorious for not being able to handle anything but very good quality burns.