is it dry milling or wet?...I suggest you can use wet milling with a suitable dispersion media. this sticking may come from van der waals force or the high speed running of the ball or from other factors...
you may need to use a surfactant during wet milling. at some point during the milling procees, the nanoparticles start agglomerate and you don't get any further particles size reduction. do some litterature research to find the right surfactant for your TiO2 particles. just make sure that the concentration need to be above the CMC ( critical micelles concentration),