  Currently, 6 candidate binary lens events have been reported, while only a
single candidate binary source event has been reported. To account for the
rarity of binary source events, Dominik pointed out that for a typical binary
source event the separation between the component source stars is very large,
resulting in large difference in impact parameters between the component binary
source stars. In these cases, the light curve of the highly amplified source
star is barely affected by the light from the other star, making the observed
binary source light curve difficult to distinguish from that of a single source
lensing event. In this paper, we determine the fraction of events with similar
source star amplifications is as much as $\sim 8%$, and thus show that the very
low detection rate for binary source events cannot be explained by this effect
alone. By carrying out realistic simulations of binary source events, we find
that a significant fraction of binary source events are additionally missed
from detection due to various other reasons. First, if the flux ratio between
the component stars is very large, the light curve of the bright star is hardly
affected by the light from the faint star. Second, if the separation is too
small, the binary source stars behave like a single star, making it difficult
to separate the binary source event from a single event. Finally, although the
probability of detecting binary source events increases as the source
separation increases, still some fraction of binary source events will be
missed because the light curves of these events will mimic those of single
source events with longer timescales and larger values of the impact parameter.
