Details
A PMI balloon can be associated to another PMI balloon without the leader in all 4 directions (3'o clock, 6'o clock, 9'o clock and 12'o clock position) around the balloon. The associativity rectangle shows up with the corresponding balloon position.
However, the same is not possible if we try to associate PMI balloon to another PMI balloon with the leader in 12'o clock position of the balloon. In this case, the associativity rectangle does not show up with the corresponding balloon position.
Solution
The reason that the second ID symbol cannot be stacked above the first is that the first ID symbol (with the leader) is being created with a Relative To Geometry associated origin. If another symbol were stacked above it, this would require the first symbol to have a "stacked" associative origin. A symbol cannot have two associative origins, so the operation is disallowed.
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: 1064
Product: NX
Application: PMI
Version: V1863
Function: STACKING
Ref: 002-8566901