Going back years ago there were some songs I tried to learn and just plain failed, not because they were overly difficult but because they were just slightly off standard tuning. Just enough to put me off when working it out and if I had a transcription, given I tend to learn things by playing along with them, just enough to stop me trying because it sounded godawful.
I guess sometimes it was deliberate, sometimes down to the mastering process or dodgy production and frankly sometimes because they tuned up by ear without being perfect at it. Who knows but they were just slightly out. I could have carefully tuned my guitar until I was in tune with the recording but frankly never could be arsed. Down or drop tuning puts me off enough already and you don't have to go fishing for that.
Nowadays I have a computer where I practice and tend to play back things on that. I got recommended a program called 'Best Practice' which allows you to arbitrarily change the pitch and speed of either MP3s or CDs. Best of all it's easy to use, open source and free. Although it looks like the developer hasn't touched it for a while, maybe some donations would encourage him, must do one.
With it you can set something playing and tweak it a cent at a time until it's spot on. Or if it's difficult to pick out, slow it down. Or both. If you really slow something down a lot or change the pitch greatly it sounds glitchy, but for the small adjustments I've needed it's been perfect. Then you can loop the bit you want to learn. You can also save the results as a .wav file for burning to a CD. I've made a couple of practice CDs that have 'corrected' material on and it's really helped.
If you're finding the same problems I did give this a shot. Windows only I'm afraid, but there you go.