If I recall correctly, you can't skip a level with Flexibility. The reason is that it creates a Family table in the assembly that the component is in so that it can substitute the altered item.
If Spring_A is in Sub_Assy_B is in Top_Assy_C then a family table is created for Spring_A in Sub_Assy_B. It can't be created in Top_Assy_C because Spring_A is not in Top_Assy_C.
I think the only way around it is to structure your own family tables for the spring and subassembly or use flexibility to suppress the springs in the subassembly and add replacements in the higher assembly.
Perhaps later versions of Creo handle this better - it was a suggestion I made a long time ago and I think it was never acted on - to add Flexibility only at the component level and have it bubble up through the assembly structure so it could be modified at any assembly level. This would require some other software approach than family tables, a method that's already in place to handle assembly cuts, so I expect that's why it wasn't adopted.
