I've had one patrol like that. Playing at 100%. I was patroling Solomon Sea and couldn't find anything. It got more and more frustrating, and for over three months at sea I couldn't find a single contact. I thught for sure that it must have been some mod I've installed but in my next patrol everything where back to normal, I even sank my first Aircraft carrier.
Ps. If you have radar installed make sure you check if it's on once in a while. High waves will turn it off, especially in small S-boats.