My hearing problem is that I can't hear voices well. Audio tests put me as exceptional hearing. Much better hearing than most. But put me in a situation with minor background noise, like an airplane, and I can't understand voices.
I'm not a doctor; just so we're clear. But, you might have more of a "notching-type" hearing loss.
Some people can "hear" the sounds of speech, easily. And only increasing volume might, or might not help, always. The thing with speech is that a consonant, or even the very first part of the sounds of syllables is actually very brief, compared to the "body" of a sound. It's about the "attack" "body" and "decay" of tones.
Believe it or not, if the initial attack, which is in the high end range of the spectrum, is muffled, briefly, then words like bay, day, may, nay, pay... etc, will be indistinguishable. We'd hear the "ay" but not the most important part of the word. Some people say, speak louder. That works, sometimes, depending on distance from the ear, background noise, etc. Some people will turn their head slightly, or even cup their hand behind their ear, and that might work far better than "simpler louder."
It's that narrow-band loss of frequency, in the high end, that explain all those partial "remedies." High freqs are narrow waves, and they bounce off (are reflected by) nearly everything. Low freqs are very wide waves that follow walls, floors, ceilings, etc. Even a guy with a very low voice (baritone or lower) has much higher freqs that are the initial part of syllables... so, we might hear 99% of what he says, but find it nearly unintelligible.
Cupping hands behind an earlobe gives the higher freqs that many more times to "bounce" and be heard, far more clearly. The reinforced sibilant makes identification of the "note" easier, even, technically, slightly "after the fact."
These things are tricky to diagnose correctly, for the simple reason that most hearing tests are done with headphones, so the sibilants and partials have direct access to the natural reverb chamber that the inside of the ear lobe provides. A good hearing specialist will be able to administer a variety of tests, including one that aims to reveal exactly this sort of loss. I'd get it checked out. Oh wait, I DID get it checked out. Was very helpful for me.
P.S. Not sure "notching" is a proper term, certainly not all pro hearing specialists would use it, as far as I know. But, think of a parametric equalizer. if a common parametric EQ divides total range of frequencies that humans hear into, say, ten "bands," then it's safe to assume each "band" includes quite a number of adjacent freqs, right? So, what the parametric part does is it allows us to narrow the range of freqs, within any of those arbitrary bands, so that only a few, or if needed, a lot, of them are affected by a 'boost" or "cut." It's a bit more "surgical" in that regard. Going the other way, dividing all the freqs into 2 parts (treble, and bass) is barbaric in comparison. And using volume, instead of EQ, just makes the same "problem" louder. HTH