It’s likely the custom client CRM has deduplication logic enabled/enforced.
You’ll have to determine if there is the ability to do one of the following with other app API endpoints:
Find else Create
Create/Update (aka Upsert)
Build beyond no-code with the Zapier SDK
Join us on Friday, June 19, 11:30 a.m. ET for a hands-on community build-along where you'll learn the Zapier SDK, follow a live walkthrough, then build your own use case with a Zapier team member in your corner.