It makes sense that you have to hit it to ask for it to be calibrated but I can't find anything that specifically says exactly that.
Appendix C1 #6 says the following...
6. If, during a course of fire, a popper does not fall when hit, a competitor has
three alternatives:
a. The popper is shot again until it falls. In this case, no further action is
required and the course of fire is scored “as shotâ€.
b. The popper is left standing but the competitor does not challenge the
calibration. In this case, no further action is required and the course of
fire is scored “as shotâ€, with the subject popper scored as a miss.
c. The popper is left standing and the competitor challenges the calibration.
In this case, the popper and the surrounding area on which it
stands must not be touched or interfered with by any person. If a
Match Official violates this rule, the competitor must reshoot the
course of fire. If the competitor or any other person violates this rule,
the popper will be scored as a miss and the rest of the course of fire
will be scored “as shotâ€. If the popper falls for any non-interference
reason (e.g. wind action), before it can be calibrated, Section 4.6 will
apply and a reshoot must be ordered.