Congrats on transcribing! Don't ever stop!!! This is some of the best training you can do not only as a bassist, but as a musician. Trust me. This stuff can be hard, but it will get easier.
What I like to do is be able to sing something in my head, then try my best to transfer it to my voice (another instrument)....once you can HEAR it and SING it, transfer it to your bass or what ever instrument you are playing.
Don't ever give up!
BTW, if you are having problems HEARING it because you can't HEAR it (due to the recording or other instruments getting in the way)....solve the puzzle. If it's a bass line, maybe it's in another part of the tune....if it's a solo - really listen deeeeeeeeeep and don't be afriad to use HEADPHONES when transcribing - this can help as well.
Good Luck!
NJL