Apex AD 1110-W playback problem

I've got Apex AD 1110-W player that has an odd problem.

I've recorded a number of DVDs (Maxell DVD-RW 4.7gb) on a separate DVD
recorder machine (a Walmart generic SV2000).

The first ones loaded and played fine on the Apex. Then, the next batch
wouldn't even load on the Apex.

I did "finalize" all the recorded DVDs. Other DVDs continue to play ok on
the Apex.

Any ideas what's going on?

Thanks