So, I'm assuming that this is when playing a tape that you've already established that channel #1 has in fact been recorded onto. If this is the case, does the VU show any signal present on that channel?
Just to double check things, get another tape and try recording onto channel #1 again.
You said below that you "checked all the leads fro the head to the plugs". Did this include a continuity test of each lead?
Also, is there some sort of switching for each of the 4 channels (not the source/tape switch). On most of these kinds of machines there's a way to control record or playback of each channel. Those would be the switches in question.