This study investigated therapist reports of client engagement challenges in delivering evidence-based practices (EBPs), within the context of a large-scale implementation of multiple practices in children's mental health services. Data were drawn from an online survey of 668 therapists (88% female, 35.0% Non-Hispanic White). The majority of therapists (75.4%) endorsed at least one client engagement challenge during their implementation of an EBP with an identified client over the previous two months. Two types of EBP client engagement challenges with distinct correlates but similar overall frequencies could be readily differentiated - Expressed Client Concerns and Limited Client Engagement in therapy activities. Limited Client Engagement were more commonly reported for clients with externalizing problems and by therapists with higher emotional exhaustion and negative perceptions of the specific EBP being delivered, whereas Hispanic/Latino therapists were less likely to report Limited Client Engagement. In contrast, Expressed Client Concerns were more commonly reported by therapists with positive general attitudes towards EBPs, and among therapists delivering a parent training EBP. Limited Client Engagement but not Expressed Concerns were linked with therapists' self-reported ability to carry out the EBP with the target client. Findings suggest that client engagement challenges are frequent during the delivery of EBPs, but may impact implementation differently based on whether they relate to challenges in engaging clients in therapy activities versus addressing concerns raised by clients.