Cancellation annoys me

I’m a scientist for bioscience in the UK. I’m studying neural system in human. So I need human volunteer to perform my experiments. Usually, I use a volunteer list which our laboratory made to recruit them as participants for our studies. Recently, I have some trouble; many participants cancelled to attend their experiences one after another. They said that they found new jobs so they will not attend them or they’ll leave from London for a few months. To fill up the vacancies, I send e-mails to the persons in the list; it is not easy to get new ones. This phenomenon happens more recently. I think that the cause of this problem may be the season; it is the end of July. The summer vacation of college has started and the life style of them changes temporarily. They get some free time to have new job. Some of them left from London and went back to their hometown. My experiments are performed on weekdays because I cannot use study devices and help of technicians on weekend. So, most of participants for my studies are college students. So they often cancel to attend my experiment and it is not easy to find another one. But I need their helps. I have a hard time of it.