That's assuming it didn't track on a noisemaker, the ADCAP isn't invulnerable to them.
Basically I'd run through this checklist:
1) Be sure of your solution, if you have a contact on the towed array then make a course turn of about 45 degrees and watch the contacts on the waterfall. One bearing will change and the other will stay the same, the one that's stayed the same is your contact.
2) Get all available information on your contact, obviously you know it's an Akula, so that'll help you put in the TPK, and firm up the Target Motion Analysis. If you're doing that manually then the only real advise I can offer is screwing with it until the dots are lined up down the center of the line. I usually leave that to the autocrew on anything other than the 688i which I can just about remember how to do from playing Fast Attack.
3) Set the Mk48 to passive mode, and check the ceiling and depth limits, and keep the torpedo wire-guided for as long as you are able to. If he fires one back at you and you have to go evasive, then switch the torpedo to active and cut the wire, otherwise wait until the torpedo is fairly close to the target and then switch it to active. It should aqquire and then home in...and then probably miss. But no problem, by now your target will be panicking and making lots of noise which will only serve to give you a better idea where he is. Turn the torpedo around and take another run at it, eventually it should hit. If it doesn't then your TMA was likely off by a significant amount.
4) Do not cut the wire if the torpedo is pointing anywhere in your direction. Another lesson learnt from playing Fast Attack when I cut the wire on an ADCAP whilst attempting to sink a Kilo, unfortunately the torpedo was pointed back at me, and when the wire cut it went after the bigger and noisier target and thus I managed to kill myself with my own torpedo...
Keep practicing, TMA is a sod, but a good solution makes all the difference. Akulas are slippery customers, and they have a nasty sting in their tails, I can't remember off hand if they carry Skhvals, I know the Victors do, believe me, getting smacked by a Skhval is something you don't forget in a hurry. The Stallion can be a nasty surprise too when it plops into the water above you, even nastier if nuclear release has been authorised (thank god DW doesn't model the nuclear depth charges for the Stallion), but I think in time you'll nail your first Akula.
Happy Hunting.