It is just like when you attack a convoy during a career. If you're submerged, you are too slow to keep up with the convoy. Normally, you have to wait until the escort(s) lose contact and head back to the convoy. By the time the escort leaves, the convoy is far away.
You need to surface and work your way ahead of the convoy to attack again. This is time consuming, takes patience, and some planning.
If an opportunity presents itself to take out the escort though, take it. You will still have to end around the convoy if you're sucessful on taking out the escort. The convoy will be well ahead of you by then.
Good luck and welcome aboard.
