This paper investigates what instructional factors influence conceptual understanding (CU) of signals and systems for senior undergraduate engineering students. Previous results show students in signals and systems courses typically gain little CU, though evidence-based instructional practices, such as active learning, can increase gains in CU. However, few studies consider CU of senior students or other instructional practices that increase CU. To explore possible factors, we interviewed two faculty members, eight undergraduate seniors, five graduate students, and four practicing engineers then analyzed the transcribed interviews using a constant comparative method. Participants identified lectures presenting CU along-side mathematical expressions; lectures emphasising purpose and connections; hands-on activities where students have control, receive immediate feedback, or where they have to apply and synthesise concepts; and repetition of concepts across multiple courses as factors that helped build CU. Grades that emphasise procedural knowledge over CU and heavy workloads were noted as hindrances to CU. This paper relates these findings to theories on conceptual understanding and previous results on factors that influence student learning.