The whole idea of the Borg is that they're in a constant state of assimilation and improvement. However, has there ever been any point in the franchise where the Borg were shown to have become more technologically advanced than they were earlier? Unlike, say, The Federation, I've never gotten the impression that the technology of the Borg has improved between their introduction and any later point in the franchise.
One example that comes to mind is the assimilation tubules. When the Borg are first shown to assimilate Captain Picard in the episode "The Best of Both Worlds", they have to take him over to their ship and carry out a medical procedure. Throughout The Next Generation TV series there is no indication that they have any other means by which to assimilate individuals.
Then in the second TNG movie First Contact the Borg are shown to possess "assimilation tubules", devices that can reach out from the wrist of any Borg drone to inject nearby victims with Borg nanoprobes and assimilate them in a matter of seconds. From this point on drones are a much bigger threat as they can quickly multiply. This goes on to be standard Borg procedure when they return in Star Trek: Voyager.
Although there are various minor instances of the Borg adapting to various efforts made by Picard and Janeway to attack them, a really solid example of technological progress is the Borg's acquisition of the Spatial Trajector from the Sikarians seen in Voyager.
The Voyager episode Drone is about a transporter mishap that causes one of Seven's nanoprobes to interact with the Doctor's mobile emitter, a piece of technology from the 29th century. The nanoprobes extrapolate various future technologies and materials from the device and create a significantly more advanced Borg drone than normal in the 24th century. (This drone is sometimes refered to as a "29th century Borg drone", but it's strictly speaking a 24th century Borg drone that has had access to 29th century technology.)
While this is not an example of the Borg developing over time, it does show that they do indeed have the capability to develop over time as they assimilate new technologies and integrate them into drones, should they come across them.
As to why Borg development doesn't seem to happen over the course of the series, here's one possible explanation:
In the Voyager episode Scorpion, it is stated that the Borg don't perform research*; all their knowledge is gained through assimilations of other cultures. As such, major technological improvements would presumably only occur whenever they manage to come across and assimilate a species with some piece of technology more advanced than their own, with is presumably a rare occurrence given how much technology they already have access to. By the 24th century, it's not inconceivable that the Borg development has effectively plateaued.
*) Though this may simply be an assertion on B'elanna's part. Later statements about the Borg performing experiments on Omega seem to contradict this somewhat.