My copies of a part do not match

You placed the same part several times and they are not behaving as one thing. One sits at a different height, or one ignores a control the others obey.

One of them is at the wrong height

Almost always this is a part that seats itself being moved by whatever placed it.

Anything that sets a position replaces what was there. So a group that puts itself on the ground, then gets moved sideways, loses the seating and drops to wherever the move says. The copies that were moved end up in one place and any copy nobody moved stays correctly seated, which makes the correct one look like the mistake.

Measured on three posts from one definition: the two that were moved sat from minus 0.6 to 0.6, and the one left alone sat from 0 to 1.2. The app flagged the untouched one, because it was the odd one out.

Take the seating out of the part and seat the finished assembly once instead.

The control does not reach

A placement sets the inputs its group declares, and only those. It cannot reach through into a group nested inside.

If a value needs to travel further down, the inner group needs an input and the outer one has to pass it along.

They stopped matching after you changed one

Placements share a definition, so changing the definition changes all of them. If one has stopped following, it is probably not the same definition any more.

The app checks that a placement's declared controls still match what the definition offers, and says so when they have drifted apart.

The part is not there at all

A placement naming a definition that does not exist has nothing to build. That is reported rather than left empty, because an empty result and a missing definition look identical on the canvas.