http://debbiejensen.com/whelping_trouble.html
Not being there, I'm not sure what is happening & as I said I don't have much experiance (only 1 litter-no problems), but check out the link I posted. This may be what is happening. I'm just not sure.
Is the mom in any distress? I think at this point I would be at least calling the nearest emergency vets to see what they say or maybe even taking her there. Two hours with a sack(s) presented with no puppy born seems a bit long to me. I'll say some prayers.....keep us posted.