ISS is from 400 to 2000km away at ALL times. Most have not checked their rigs for frequency accuracy in years. The ISS has over 6 KHz of Doppler during the pass. This same signal cannot tolerate any frequency offsets or the signal distorts as it goes out of the passband.
Modern rigs have very tight filters.
Good for ground, Bad for Space. Tune your rig in 1 Khz steps. Most FM rigs wont tune such small steps. TNC's connected to the Discriminator and Modulator produce equal level tones and the tones are independent of the speech processing circuits. Your objective for success should be to get one packet through.