The builders have failed on this twice now, so I thought I'd ask around :

There's a gap between the back door frame and the stone wall. About 1/2",
filled with cement, foam, plastic in various places.

Originally that gap was covered by a cement (sand/cement?) fillet/wedge.
That fell off.

Next try was some soft putty-type stuff. It's still quite soft after several
months, it looks somewhat like yellow sand apart from being obviously
puttyish. Applied in the same fillet/wedge as the cement, it lasted a little
longer, but now it's falling off.

Neither of these have managed to adhere to either the stone or the wood
sufficiently.

So, my question is what to try next?

People mention mastics - unfortunately a) I don't know how big a gap they'll
cover and b) what mastic actually is. So can you aim your answers at a
fairly low level of knowledge :-)

TIA,
clive