This pretty recent phenomenon can often be found in enterprise settings where the same (too) small user pool or testing group is approached over and over again. After a while they refuse to take part in new studies, especially in B2B contexts. It is therefore important to strategically build a sustainable pool of users and maintain good relations with them.
Participant Fatigue
/pɑːrˈtɪs.ə.pənt fəˈtiːɡ/