The Lamb-shift polarized ion source has been shown to be able to produce only 2–3 μA of polarized beam current. The mechanism that is responsible for this limitation has proved elusive over the years. In this paper, we present an explanation for this limitation. We have conducted a proof-of-principle-type of experiment that verifies our hypothesis. The results from the experiment are then used along with well-known properties of the source to predict the limiting current within a factor of two of the observed value.