Wolves of Steel - can't lock on target

[Ekaton]

I can't lock on target in periscope mode on the periscope depth. I was able to do so using the UZO but the sea is too rough to use it to determine the range accurately. If I can't lock the target I can't use the stadimeter and so I can't do anything. Any suggestions?

Reloaded the save (hate that) - now I can't lock the target with the UZO, though I can do so after the "ship sighted" message. I can clearly see the ship, even its flag, but still can't determine its distance because of some stupid game mechanic? What gives?!