Usually if something can only be heard on sonar I'll let it go if all 3 of these conditions are met:
-Long Range
-Moving Away
-Moving fast
If any one of those 3 conditions are not met, the hunt is on! If they're closing, I use the Hydrophone Interception technique (link below). If they're moving away, I go ahead full on the surface to try to close the distance, keeping an eye on the sonar bearing to figure out which way they are headed. Look at the map for bottlenecks or ports they may be headed toward, then head that direction to try to intercept them there.
Also, you're not heading straight in the direction of the contact are you? That's an easy mistake to make, and it almost always puts you on their tail, which is the opposite of "ambush." The trick is to make calculations to get ahead of them so that they cross your bow without knowing you're waiting for them there.
There are many good tutorials on here for how to do this. Here are a few of my favorites:
Hydrophone Interception - This is for when you can hear them, but can't see them. It works best if they're closing instead of moving away because you have to sit in the same spot for a while. The tutorial takes it to the extreme and even fires torpedoes without looking at the target. I just use the concept to help me intercept a sonar contact, then use other techniques to fine-tune my position for the kill. This technique is very helpful and worth committing to memory.
Convoy Interception - Not just for convoys... I use this technique nearly every time I attack. It works especially well with visual contacts, and can even help you decide if it's worth chasing a target from periscope depth, or if you need to surface to get far enough ahead then dive for a proper ambush. Commit this one to memory. Practice it over and over until you can do it instinctively.
- I haven't used this technique because I don't have radar yet, but I watched the tutorial, and it's done very well.
SHIV Guide - This thread has a lot of really good general info. Read through it every now and then to keep proficient.
I have had a lot more success in the shipping routes (I asked the same question about a week ago.) The Luzon strait is pretty fruitful (playing last night I got 2 merchants within 24-hours in-game time). I got one in the East China Sea too... didn't take long after I got there.
Happy hunting!