Improving equity of education funding is a key to addressing social injustices in education. Weighted student funding systems are a relatively novel approach a number of large urban and suburban districts have used to try to improve the equity of school funding. In a weighted student funding (WSF) system, the dollars distributed to schools are "weighted" to provide more dollars to schools enrolling high-need students, such as those who are economically disadvantaged, English learners, or students with disabilities. Using an interrupted time series approach, we examined trends in equity over time in four WSF districts to examine whether equity improved following WSF implementation. We find that equity of resource distribution improved meaningfully as measured by both the progressiveness of funding with respect to poverty and the predictability of funding in only one of the four districts examined in the longitudinal analysis. We performed several follow-up analyses, including simulations of WSF formulas, to understand why we did not observe improved equity in WSF districts. We show that distributing dollars outside of the WSF system, charging schools average rather than actual salaries for teachers, the use of weights that reward lower-poverty schools, and the provision of extra funding to magnet schools or other lower-poverty schools with specialized academic programs can all work to erode equity gains in WSF systems.