I've had some problems with some of my music videos coming out with the volume too low, which led me down the rabbit-hole of volume normalization.
YouTube and other sites decrease volume on audio based on the "loudness" of the signal as measured in LUFS. While my mixes don't sound especially loud to me, I suspect that the hard-limiting I do (basically just flattening peaks that exceed maximum amplitude) is affecting the loudness calculation.
Going forwards, I'm going to try reducing volume so I don't need to hard-limit and then using "matchering" to remaster the track to have volume/eq profile similar to a known-good track.