You don't say how, or if, you normalized the audio on the computer.
There are lots of potential adjustments and/or problems at the tape recorder, and the playback of the tape.
For starters, I suggest connecting a set of headphones to whatever sound card output (line out?) you are using. Check the volumes there. That will tell you if the problem is in the computer, or further downstream in record and playback.
I've been converting old, old LPs and tapes using audio cleaning Lab 3 with great success for about three years now. The CDs I've put together are sounding good at the ice skating arena, where the sound system reduces everything to mono.
Tapes are better for some users but I have a problem with the volumne. The first tape I did sounds as if it was never volume normalized.
Is something lost when going from the computer to cassette tape? The sound when actually recording, with a line in to the cassette recorder from the computer, is fine but the playback from the cassette has some songs way too loud and others barely audible.
I believe the sound card is from Crystal Sound Fusion.