I've consulted the VDC article on this (Looping a sound) and tried two of the programs there, LoopAuditioneer and Goldwave.
LoopAuditioneer lets me create loop points, for which I can input precise sample numbers, but which Source just straight up doesn't pick up on. The entire file loops. It also lets me create cue points, which I can only create by vaguely clicking into the waveform with the mouse without any sort of sample precision.
Goldwave lets me create cue points by inputting specific timecodes, so that's what I did. I used their online version, since I didn't want to bother with the trialware. I named them as the wiki shows (start_cue and end_cue) and saved the file.
The entire file just loops from start to finish as well.
My files are 44.1 kHz 16-bit PCM WAV files, which is within the specifications the VDC wiki outlines ("Source supports uncompressed 8-bit and 16-bit PCM"). I used a hex editor to verify the appropriate "cue" chunk has been written into the file by Goldwave.
I'm a little out of my depth here and at my wits' end; I've followed the tutorials letter for letter and it still doesn't work. What am I doing wrong?