As the climate warms, many species are flowering earlier in the spring. This could be beneficial, allowing plants to still flower during the historical climate conditions needed for successful reproduction. However, it can also have unintended consequences, potentially changing interactions with pollinators or risking flower damage from unexpected frosts. Few studies have evaluated whether shifting flowering phenology is sufficient to maintain reproductive success under climate change, or the consequences of variation among individual plants in the magnitude of flowering shifts. We use a rare, individual-level dataset on the flowering phenology and reproductive success of an alpine plant, Silene acaulis (Caryophyllaceae), over six years spanning a wide range of snowmelt and spring temperatures. We show that, like many other alpine plants, S. acaulis dramatically advances its flowering in response to early snowmelt and high growing degree days (GDD). However, earlier flowering is insufficient to track historical climate conditions or maintain successful reproduction. Further, individual plants vary dramatically in their phenological responsiveness to climate, in ways that impact population-level fitness. Together, this work demonstrates that even large phenological shifts in the appropriate direction may not buffer populations from the effects of climate change.