We too have a new puppy (12 weeks old) and accident free! (for about 2 weeks). We also had 2 other dogs before.
First you pretty much are doing everything right. Everydog is different. Our first dog was house broken the first week (that is really odd) our second dog still had accidents in the house at 5 months! He would go out side and come back in and do his business and look right at you! His was more of a behavior issue (was mad we left him alone!).
As the previous poster said you need to be looking for the clues. Also, I would recommend crating him while you are gone. Leaving him in a big room will allow him to mess in a corner and sleep/play in opposite corner.
Just make sure he doesn't have too much room in the crate (he should only be able to turn around). If there is extra space they will eliminate in the corner. If you can afford to have someone let him out 1/2 way through that time until he gets another month older that would be even better, it will elimate anxiety and he will be ok with other people taking care of him.
The dog is a little young to be in a crate for that long. However, our first 2 dogs were in there crates for 10 hours a day/5 days a week until about 9 months old and then they had the laundry room. They were ok (ie. no anxiety) but were very active when we came home.
Also, make sure you remove his collar in the crate (so it doesn't get caught in the grate) and only put a safe toy in the crate for chewing (ie kong ball) unless he is an aggressive chewer then don't put anything in there.
Any trainer will tell you the crate is the safest place for the dog and they find it very comforting (close quarters). And it is good to crate train in case you need to board your dog or if they have surgery etc. You just have to make sure it is not viewed by the dog as a punishment. All of our dogs always went to there crate to sleep, even during the day.
Pups are very similar to having a toddler, they have seperation anxiety, potty issues, chewing issues etc! Now if I could just get my pup to quit nipping at my son! (he thinks he's part of the litter!) Good Luck to you!