[Dogfish40]

I noticed that happening when visibility was an issue, fog or night time tracking would lose the periscope lock until you closed on the target. I was suprised at the fog because, although I could see the ship quite clearly, the game evidently thought otherwise and I would have to keep relocking the target. I now keep the "volumous fog" option on in the graphics so I can have a better idea of the weather conditions.
D40
PS: the other obvious times are in heavy seas, when the waves keep washing over the scope and then it loses lock easily...