My part is floating or sunk

A piece hovers above where it should sit, or half of it has disappeared into the thing below. It usually looked right when you built it, which is the clue: something was measured once and then a size changed.

A typed height only works at one size

If the position came from a number you typed, it was correct for the size the shape happened to be at the time. Any knob that changes that size breaks it, and nothing reports it because a position is never wrong on its own.

Replace it with Place On. It measures both shapes each time it runs, so it holds at every size.

Something after the seating moved it

Transform sets position outright rather than adding to it, so a Transform downstream of a Place On throws the seating away and puts the shape wherever it says.

Seat last. If you need a Transform as well, put it before.

The shape sits on a point rather than a face

Seating measures the lowest part of a shape. If you roughened, displaced or bent it after seating, the lowest part may now be one stray corner, and the shape balances on it while everything measuring the shape still reports sensible numbers.

Either seat after the roughening, keep the underside out of it, or flatten the base afterwards.

The two parts never found each other

Attach works by matching names. If the names differ, or one part has no socket at all, there is nothing to match and the child stays where it was rather than moving to a wrong place. The app names which side is missing.

Check both spellings first. Leaving one name empty makes it mirror the other, which is the usual fix when two families use different words for the same joint.

It attached, but the wrong way round

A socket can be marked as providing or accepting a joint. Two that both provide will not pair. If a join refuses for no visible reason and both names match, that is the thing to look at.