As I’ve stated in response to the same issue found on the console version, and found today on PC:
Reviving this thread for S4 (5/19). I have tested this on mobs/bosses as well as with/without the Unique Item (Mutilator Plate). The buff is only gained on bosses and in no other instance. It even works on the boss test dummies but no other dummies (both elite or multi target). This is in effect on both PC and Console it seems.
P.S. tooltip for reference:
" While at least 2 enemies or a Boss are affected by Blood Lance, you gain 15% [+] Attack Speed and Blood Lance’s Essence cost is reduced by 3."
Feel free to chime in if you have experienced this bug as well or found out what may be causing the issue.