Single note won't play - in bass stave only

I cut and pasted a note-sequence from Tenor to Bass. One note refuses to play - in the Bass stave only. It isn't to do with its high pitch - I have tweaked the range to make it acceptable and it makes no difference. I think I must have hit Caps-Lock or something by accident whilst adjusting the phrase's pitch back and forth, but can't reconstruct what I did. It played immediately after I cut and pasted it, but after adjusting pitch then putting it back, the F won't play. I have tried deleting the bar and reinserting, no joy. Likewise deleting and writing out the phrase from scratch. Nothing works.

Ideas?