It is usually not very difficult to engage sites who have been chosen to participate in a clinical study at the beginning of the trial.  This is an exciting time for any site that is able to bring that new experimental drug or device to their patients who are able to participate in the study!  Many times, during the enrollment phase of the study, sites continue to be excited and active in their roles, and this is usually facilitated by sponsors who are actively encouraging their sites to enroll subjects.