The deuterium-deuterium (D-D) fusion reaction rate in a spherically convergent ion focus is observed to significantly exceed the rate predicted by a collisionless flow model. However, a careful consideration of ion-neutral collisions and the trapped neutral density in the cathode account for the extra reactivity without invoking anomalous ion trapping in the converged core region. This conclusion is supported by proton collimation measurements, which indicate that the bulk of the observed reactivity originates outside the core region. In addition, a classical flow model, where charge exchange collisional effects on the ion and fast neutral distributions are included, provides fusion rate estimates that are quantitatively consistent with the observed D-D fusion neutron production rate.