Just keep trying you will catch one eventually. There are many reasons why you would miss them, course changes, variable speeds etc... let alone the fact that the ocean is big and you have to be within about 10-15 miles to see them (in daytime good weather). You can hear quite a lot further underwater on your hydrophones I think maybe upto 30-40 miles?
If you get where you think they should be and you can't hear or see them, try to take into account shipping routes i.e where would they be going that may give you a clue. Sometimes you might be in the right place just arrived too quick, in which case you should wait, other times they might have increased their speed to 20+ Knts and left you in their wake.
I remember it took a while but eventually I caught one, from there you just get better at judging where they are going (to be) and it becomes easier.
