Depends on engine version.
UE4/5 without Chaos - never works with external collisions. Only internal PHAT.
UE4/5 With Chaos - May work with both.
Make sure you have the Phat asset assigned to the cloth.
There’s a hard limit to how many collisions affect cloth. And realistically, if on PhysX you should download and build the ApexTools from scratch.
https://gameworksdocs.nvidia.com/APEX/1.4/docs/APEX_Clothing/ClothingTool.html