A person can be available at the right hour and absent in every other sense.

Presence is not the same as availability. Someone can be reliably there at the agreed hour, do everything correctly, and still be absent in every way that mattered, and the accuracy of their attendance makes the absence hard to name. There is nothing to point at, which is why this particular loneliness is so difficult to describe to anyone else.

If you feel unmet by someone who is technically doing everything right, you are not being unreasonable. Try to name the specific thing missing, since attendance can be requested and attention has to be asked for differently. And check your own attendance too, because it is easy to be reliably present and quietly elsewhere.