Assembling a huge assembly into a new assembly does not show all sub-assemblies and components.
Solution
Please check if in the Assembly Occurrence Properties of the added assembly the "Higher Level" for the not displayed sub-assemblies and components is set to "No".
See extracted information from the Solid Edge Online Documentation:
http://docs.plm.automation.siemens.com/tdoc/se/2019/se_help/#uid:xid1129777:xid1323753:index_assemble_parts:xid280374:prtprp1dHigher Level
Specifies that the occurrence is displayed when the current assembly is used in a higher-level assembly. When set to No , the occurrence is not displayed in higher-level assemblies and is not included in a parts list when the higher-level assembly is placed in a draft document. Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: 1064
Product: SOLID_EDGE
Application: ASSEMBLY
Version: V219.0
Function: COMPONENTS
Ref: 001-9478244