How do people manage to keep these things alive?

I've tried in 2 out of my 3 tanks and they have consistently failed to
thrive. They sometimes manage to hang on there for up 2 weeks, but
they always end up dead.

In one tank a 55g with an oscar and a plecostomus, they just simply
disapear. I have checked to see if they are just out of sight during
the day by looking with a flashlight during the night to try to find
any - no luck.

My other tank is a 72g that is heavily planted and has onyx sand as
the substrate (for comparision the 55g has a neutral substrate). In
this tank, oddly, the snails seem to hang on for a small period of
time, but their shells end up lightening in color (they start up a
dark brown, almost black) and I find them on the surface of the sand
dead. The onyx sand should be buffering the water to provide more gH
and kH correct? Shouldn't that be helping the snails out?

Lastly, my 3rd tank a 20 gallon has a profusion of Malaysian Trumpet
Snails, the tank is rarely fed and the gravel is neutral. I do not see
why this tank would differ from the 55 in chemistry as the water
changes occur at the same time and frequency - ie. the water chemisty
is exactly the same.

Do I lack suffient minerals in my other 2 tanks for whatever reason to
keep them alive? I know it is not food as I feed profusely and I know
that there is extra food. I even try leaving an extra algae wafer for
them when I do introduce them. They just won't stay alive.

My tap water is pH 7.2, gH 3, kH 5 - no other chemicals like flouride
etc, as it's well water. Do I need to add something to my tanks to
have the snails survive? Everyone seems to be able to keep them alive
except me. What is more frustrating is that they breed like crazy in
the 20 that I rarely feed and I routinely pull 30-50 adult snails out
of there to introduce to the other tanks.

I do not understand what I'm doing wrong.
Any suggestions on what I can do that might help them out? I really
want them to survive in my other tanks if at all possible.
I've racked my brains and all I can think of is I need more Ca, but I
cannot understand why they do well in a tank with the same water, but
do not do so well in a different tank.

Any help would be appreciated.