Take a look at your speaker system's spec sheet... take a look at the angle of horn coverage. Now, imagine that the horn projects light instead of sound. Where are the dark areas in the room? Will people be sitting there? There will be an area in the middle where BOTH horns can be seen well, and many others where only one will. If you set up so that the 'sweet spot' in the middle is great sound, those that only get to hear one of the horns (if that!) are going to get poor vocal intelligibility. Not so much when you are singing, but a lot more when you are talking.

Add to that, those in the 'cheap seats' can't see your mouth as well (it's amazing what the mind can 'hear' when it 'sees' your mouth move!) and quite a bit of understanding about why the people in front can somehow hear you but many can't becomes apparent.

But, short of a better PA, the main thing that will help you be understood between songs is projecting well. Even though you have a PA, you aren't chatting over your kitchen table... you are STILL addressing a room full of people! Talk clearly, don't mumble, make your jokes count, don't toss them off, you'll find your audience understanding you a LOT better...

When in doubt, tape your show with a mike out in the room (not direct from the board). Listen for yourself to what your audience has to deal with!