The recent TNG novel Greater Than the Sum
by Christopher L. Bennett establishes that there are two types of drones -- those assimilated from pre-existing individuals, and those grown by the Collective itself in its maturation chambers. The drones that are grown by the Collective are genetically engineered to be asexual. Greater
establishes that the drones aboard the cube encountered by the Enterprise
-D in "Q Who?" and "The Best of Both Worlds" were the genetically engineered asexual drones and that the drones encountered in First Contact
and later episodes were assimilated from pre-existing individuals.
The novel explains the shift from one type to the other by establishing that during the war with Species 8472, the Collective lost a huge percentage of its asexual drones, and engaged in a large-scale campaign to replace its lost population by assimilating numerous cultures throughout the Delta Quadrant, thus causing a large "demographic shift" in the Collective's population during the 2370s.