You can only reroll any die one time.
From page 12 of rules
'reroll: Some effects allow players
to reroll certain dice. To resolve this,
the player picks up the appropriate
number of dice from the common
area and rolls those dice again.
important: When a die is changed
or rerolled, ignore its original result
and apply only the new result. This
new result may be modified by other
effects; however, a die that has
already been rerolled cannot be
rerolled again during this attack.
Does spending a Target Lock after re-rolling a blank allow another roll? If not, I've been playing that incorrectly.