My turntable is connected but sounds very quiet and thin. What am I missing?

You are missing the phono stage. Faint, thin and bass-less is exactly what an unamplified cartridge sounds like through a line input, so nothing is broken.

The why, briefly. A cartridge outputs a tiny signal, a few millivolts against the roughly one volt a CD player or streamer sends, and the tonal balance on the record itself is deliberately altered at cutting time (bass reduced, treble boosted, the RIAA curve). A phono stage does two jobs: amplifies that tiny signal massively and reverses the curve so the balance comes back. Skip it and you get a whisper with no bottom end.

The fix is one of three, and only one of the three:

Your turntable may have a built-in phono stage. Look for a small switch on the back marked line/phono, and set it to line. Then it plugs into any ordinary input on the amplifier.

Your amplifier may have a dedicated phono input, marked exactly that on the back panel, usually with a ground screw next to it. Connect the turntable there and nowhere else, with the turntable's own switch on phono if it has one.

Neither? Add an external phono stage between them: turntable into the stage, stage into a line input.

Now the warning in reverse, because this is the next mistake. Turntable switched to line, plugged into a phono input, means the signal gets amplified twice. The result is horribly loud distortion and, at volume, a risk to your speakers. One phono stage in the chain, never two, never zero.

While you are behind the rack: connect the turntable's thin ground wire to the amplifier's ground post. That is the cure for turntable hum, not an optional extra, and it costs nothing. If the sound is now full and there is still a hum, that wire is the first suspect.

Still thin with all of that right? Send us the turntable and amplifier models and a photo of the back panels; the answer is usually visible in the photo.