We investigate the dependence of four-wave mixing response on the photon energy close to the fundamental exciton (X) resonance in GaAs quantum wells. We find that cross-polarised incident fields give rise to a non-linear signal which decays faster at energies below the X line centre than above. We show that this behaviour cannot be assigned to biexciton transitions alone but rather suggests that the delocalised X states above the mobility edge are excited off-resonantly by the laser light having slightly lower energies.