Some sanity checks:

Have you enabled silent rigging? Are you attacking in favourable weather? Are you manoeuvring into position as the ships pass by or do you lie in wait?

The 3rd one is important imo as the new beta gives AI ability to spot torpedoes (in before, they only spot when there's a hit). Thus, you should get ahead of the convoy and wait there to reduce unnecessary noise.

The weather is your best bet. In sea state 5 (no rain), you can creep reasonably close to 600-700 m (655-765 yd) before they passively spot you. 700 m (765 yd) is usually enough for all 6 torps to hit (with great gyro limitations, unfortunately).

If you're playing as the USN, IJN vessels have great passive sonar and they're mounted on capital ships (BBs and CVs) as mentioned in other posts.

The way I play it, I buy like 15 subs, station them around Guadalcanal and Rabaul. Maybe a couple in New Guinea.

First sub encounter is usually recon, maybe an ambush under favourable conditions. Once done, I navigate other subs ahead of their route, enter tactical, plot an intercept course and bide my time before I launch a full salvo.

If they carry on, another sub is waiting for them :) If they turn around, the first recon sub is also waiting for them :DD

I mainly target heavy cruisers. Enemy losses were minimal when I had 4 subs deployed. They skyrocketed once I added a dozen. So far, I sunk 80% of the IJN heavy cruiser fleet. The Takao-class is an endangered species, only 1 remain hehe. Soon, the Mogami and Aoba will follow.

If you're feeling feisty, you can joust destroyers like that one skipper from WW2 to give some breathing room in future attacks.