Long post for simple questions. Sorry.

Q1.

Amongst the many little faults that we're noticing around our new (old)
house is that the plaster is falling off some of the stud wall nails.
The house is over 30 years old so I have no idea if it's suddenly just
happening or it's an old problem that has been patched and is happening
again.

If I push on the boards where I can see the plaster coming loose the
board will give a little and the plaster will come off. Is this
usually a case of the nails not being covered over properly in the
first place or have the boards not been put up correctly?

I presume just patching it wont help as anyone hitting the wall will
just cause the plaster to come off again. Will it be worth it to
hammer some new nails into the parts of the wall that give? Or just
knock the existing nails a bit further in?

Q2.

The plasterboard on the staircase had a whopping great hairline crack
running along it when we moved in. I can't say if it was there when we
viewed as it seems in hindsight we must've viewed the house with our
eyes closed. However I think something (possibly a heavy mattress) has
bashed the wall hard enough to cause cracking but no dents. The crack
is both horizontal and vertical and seems to follow the staggered
pattern of plasterboards.

Anyhoo, I think it might be a little bit of a bugger to fill but from
things I've read about plasterboard walls one thing that pops up is
taping the joints etc. If you don't tape them you'll get cracking in
years to come etc. I presume that the boards weren't taped. Maybe
they weren't supposed to be taped. However, if it's filled will there
be a chance of the crack reapearring in a few years time. It's a canny
size (comparable to the full length of the big boards you can get and
then a foot or so up and down from it) to have to patch up every so
often.

I say it's hairline but my other half is insisting it's getting bigger.

Cheers

Anth