Have you tried to stand back a bit further and zoom in a bit more ?
I think to get a nice shallow dof one should to get to a distance where you could use the zoom to maximum effect . Maybe also try to use the manual focus .And how far are the houses from the tree you tried to focus on ? What i also experienced is to get as much space as possible between the object you want to shoot and the background .
Hope this helps a bit .
Regards
james
Originally Posted by outofline