If all of your overlays look good, and you see the estates getting all the right services, it could be that Entertainment is the "hidden" enemy. There's a great thread about Entertainment that gives all the details, but if you missed it here's a quick brief on how this could cause your devolution.
If your city has good enough coverage (I think "Perfect" is needed) in one or more areas (Juggler, Dancer, etc) then your entire city gets some bonus Entertainment Points. This might be enough to make the estates evolve. Then, all those extra people move in, which increases your population, which decreases the Entertainment Venues-per-person ratio, which causes you to loose those bonus points, which causes the estates to devolve (whew!). Since it could take a long time for imigrants to move into those upgraded estates you may not even notice why its happening.
Then, as others have said, if the estate devolves away from your road it will keep on a downward spiral to nothing. Of course, as this happens and everyone is leaving your Entertainment ratio goes back up. Once service access is resored to those houses they may shoot right back to estates only to devolve again in a year or so when those hundred and some odd people move back in.
Just my 2-debens worth. Good luck!------------------
Lawrence Tilly
New Hampshire AnlaShok
"We live for the code
We die for the code"