The family is set for "breaks into" for part type. When it inserts on a duct, it inserts and moves. When it is off of a duct in open space, it stays where it is first inserted.
Need more info. What family is it, where did you get it? If you built it, did you start from scratch, or modify another family? Where is the insertion point in the new family?
It is a family I am making. Modified a previous familt that was started from scratch. Cannot/do not know how to turn on, look for or set insertion point or coordinates. Tried looking in help, but too hard to find.
I've seen this many times. The default insertion-point (of a family element) is where the Front/Back, Left/Right, and Ref-Level reference planes meet. I will bet that your Ref-Level is at the bottom of your new family geometry. If you want it to "break into" another element (the duct) the center-lines of both elements need to match. You will have to open the new family in elevation views and move the geometry to the centerline.
Tip: If you have changed categories (original family was Mech Equip, and you changed it a Duct Accessory) you may have corrupted the file. Just start over with the correct category, or you will fight this forever.
All good stuff. It is a simple rectangular box. All of the reference planes are at the center with equaled reference planes. The elevation plane is at the center. It is set for "breaks into", but I am no expert with this stuff. I tried it and it worked. It does insert without moving when I bring one into plan. When I put in on a duct, it shifts. The planes have "Defines Origin" checked.
I has something to do with "constraints" and/or "offset". Saw this recently. Some geometry from an existing family was pasted into the new family. When you paste geometry, parameters that are associated with that geometry come along with it. You can end-up with an offset, looking for a plane, that doesn't exist in your new family. Programmers call these unassociated parameters "open threads". They are a common source of bugs in any software. When the new family comes in contact with an element that (by chance) may have the plane it was looking-for, you get "un-expected results".