Confronting someone about their weight is an embarrassing and uncomfortable thing to do. It isn't surprising that agents and F/As are reluctant to take the initiative to do so. Often they will need a nudge from the affected passenger(s) sitting next to the customer of size.

As a passenger, knowing the policy, and that it is the COS that is to be moved, not the passenger who's space is being compromised.

As always, if the problem is not being resolved according to the policy, request the CRO (Complaint Resolution Official).

From https://www.transportation.gov/individuals/aviation-consumer-protection/what-do-if-you-have-problem
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When should I contact a complaint resolution official (CRO)?

• If you encounter a disability-related issue related to an airline accommodation or service, you can request to speak with the airline’s Complaint Resolution Official (CRO) or a supervisor. A CRO is the airline’s expert on disability-related issues in air travel and has the authority to resolve complaints on behalf of the airline. Every airline must have a CRO available either by telephone or in person during operating hours.