Inconsistent DVD play

Sometimes I can get DVDs to play and sometimes not.
In some cases, nothing has changed except that a
DVD I played a few minutes ago will not play on my same Mac.

Sometimes when I start the player, the DVD
menu will appear. Other times, for no apparent reason, the
only thing to pop up are envelopes ("Audio", "Video", ec.)
which I can't seem to use to play the DVD.

At present I am using an exterior DVD player connected to my Mac
but I had the same problem when I used my Mac's DVD
player.

Sorry for my poor manner of explanation.

Thanks in advance for your thoughts, people.