It is sometimes difficult to implement even the most carefully planned and prepared policy. Wording problems may not show up until the policy is tested under actual circumstances. Situations that the policy was designed to address may have changed while the policy was developed and it is no longer relevant.
Unanticipated situations may have arisen that the policy does not address. Resistance to controversial policies may be stronger than expected and the policy can't be enforced in its original form.
All of these — and other problems — can prevent effective implementation of a policy. Accordingly, policies must be monitored throughout their lifetime but most importantly when they are first put in place. This is the time when most implementation problems will appear. Catching and correcting problems as they surface makes implementing policies much easier over the longer term.