Is the entrance hole easily accessible from the tree trunk? If it's high enough, 10' or more and faces south, can't think of any other reason. Possibly there are enough natural homes for the population in the area?

Yes, this is the old Gardenweb site, greatly cheapened.

Is the house leaky or drafty (poor seams, an unnoticed hole, etc.) or has it been stained or varnished? Those might put the squirrels off...

It is built just as I have seen the plans here and there. I ordered from a squirrel website, instructions and a lot of information about squirrels. No paint or varnish, the squirrels run in and out but no one wants to live there for some reason.
I see nests in the neighborhood trees.
Just one of those things I guess.
I like the old Garden Web.

Don't give up, I'd bet sooner or later the right Squirrel will come along & make your Squirrel house it's home... :-)
Good luck!

I want to second dreamweaver! I got my dad a bat house and for years I asked him...got any bats yet dad? The answer was always no. Last month I was helping him clear invasives in his timber, we were beside the garage with the bat house and I hear this squeaking noice. I walked away and just kept getting closer and closer, till finally I can tell it's coming from the bat house. I tell him he has bats and yes he knows...he just forgot to tell me! I bet it took 8 or so years for them to finally take up residency! Don't give up! Some day some squirrel will want the house you put up for its own.

Terry

Thanks for the encouragement. I hope someday to get the critters to move in.

Maybe when nesting season comes around not only will they live in your Squirrel house but reward you with babies too... :-)
I feed the Squirrels & would love to have a Squirrel house but afraid it's not in my budget for now but hopefully will be some day.

I have had many babies in the neighborhood at my feeder. The very first squirrel I had was just a baby and hardly taller then the mowed grass lawn. He had been in an accident and had a bloody stub tail. He came in with his mom and ate at my feeder several times a day and played all around the tree and on the squirrel house. "Stubby" got well, grew up and stayed all winter. In the Spring mom chased him off and he wasn't at the feeder anymore. I grieved for the little thing and missed him, I thought he had probably gotten his pants pressed on the highway. A year later "Stubby" showed up with his companion and sat on the neighbors fence like he was showing her where he grew up. An acquaintance I know said they are living downtown and he has seen "Stubby" around their neighborhood a lot.
With a beautiful name like "Dreamweaver" anything is possible and I hope a squirrel house comes your way soon.

Our boxes have faced east, north, and south, and the two we've got up in the maples this year face N and E.

There's the option of contacting your local wildlife center and get enrolled as a suitable release site for eastern gray squirrels, or the kind of critter you prefer!! Wildlife rehabilitators are always looking for property owners who have ideal or great habitat for their fostered critters. The furries released by us a few months ago have stayed but they arrived as babies in pre-made nestboxes from the wildlife center. (hop to my website for a visual from last January.) Or next time the ladder's handy, can you climb up and peek inside to make sure they ain't grossed out by the interior decorations? Put a cache of nuts inside the box, see if that'll entice some residents? Good luck, I hope someone will move in and feel safe and secure in lieu of building a drey in the trees. The drey can be a great summer cottage, but nice to have a solid roof come wintertime!

(Last year Mommma squirrel thrilled us by raising two litters. My hubby removed the old ones from the trees this fall. Discovering that they were filled w/earwigs and yuck.)