Flight deck window shows only narrow part of the sky - and you mostly look horizontally. You will need to look upwards to get the ISS at its brightest.
Additionaly, ISS orbit is low and possible spotting time (darkness on ground, ISS sunlit) is short - effectively one or two passes on evening and another 1 or 2 on dawn. Winters gives slightly better chance.
Nevertheless, spotting ISS from flight deck should be possible. Check ISS ground track before flight or use some portable software which shows the ephemeris live.