  Various quasipotential two-body scattering equations are studied at the
one-loop level for the case of $t$- and $u$-channel exchange potentials. We
find that the quasipotential equations devised to satisfy the one-body limit
for the $t$-channel exchange potential can be in large disagreement with the
field-theoretical prediction in the case of $u$-channel exchange interactions.
Within the spectator model, the description of the $u$-channel case improves if
another choice of the spectator particle is made. Since the appropriate choice
of the spectator depends strongly on the type of interaction used, one faces a
problem when both types of interaction are contained in the potential.
Equal-time formulations are presented, which, in the light-heavy particle
system corresponding to the mass situation of the $\pi N$ system, approximate
in a reasonable way the field-theoretical result for both types of
interactions.
