We report on national trends in STEM program quality using the Dimensions of Success (DoS), an empirical observation tool that provides a common definition of STEM program quality. We analyzed ratings for 12 dimensions of quality obtained from 452 DoS observations performed in 452 STEM-focused OST programs across 25 U.S. states by certified DoS observers. When plotted on a graph, the averages for the 12 quality dimensions display a 'double-dip' -- a phrase that has been used in practice to communicate OST STEM strengths (higher ratings) and challenges (lower ratings). Nationally, OST programs excelled in quality indicators related to features of the learning environment, including preparation, materials, and space, as well as relationships. However, programs demonstrated less consistent evidence for quality in dimensions related to STEM knowledge and practices, including STEM content learning, inquiry, and reflection (dip #1), as well as areas related to supporting youth voice and STEM relevance (dip #2). This 'double-dip' persisted regardless of region, locale, season, and participant age or gender, though certain program and participant characteristics changed the magnitude of the scores. Ongoing professional development efforts are needed to address persistently challenging areas that are essential for building children's STEM skills, content knowledge, and fluency.