External library in extension package composer.json "no matching package found"

[Spaaza Magento Extension]

Hi,

I'm trying to upload an extension package to Marketplace and keep getting a failed Installation & Varnish test because the process can't find one of the required packages in composer.json in the extension package:

./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for spaaza_crm_and_loyalty/module-loyalty 1.7.3 -> satisfiable by spaaza_crm_and_loyalty/module-loyalty[1.7.3].
    - spaaza_crm_and_loyalty/module-loyalty 1.7.3 requires spaaza/php-spaaza-api ^1.4 -> no matching package found.

The relevant bit of composer.json in the extension package looks like this:

    "repositories": [
        {
            "type": "git",
            "url": "https://github.com/Spaaza/php-spaaza-api.git"
        }
    ],
    "require": {
        "php": "~7.1.0||~7.2.0||~7.3.0||~7.4.0",
        "ext-json": "*",
        "spaaza/php-spaaza-api": "^1.4",
        "magento/framework": "^102.0",
        "magento/module-directory": "^100.3",
        "magento/module-backend": "^101.0",
        "magento/module-store": "^101.0",
        "magento/module-customer": "^102.0",
        "magento/module-tax": "^100.3",
        "magento/module-quote": "^101.1",
        "magento/module-sales": "^102.0"
    },

The package version 1.4 definitely exists in Github and it installs when I run Composer locally. Is there something I need to do to get it to install the 3rd party package?

Thanks in advance if anyone has any ideas!