Users aren't redirected to the Identity Provider login page.
SAML configuration hasn't been assigned to RAS Theme or users access RAS HTML5 Gateway not using the appropriate theme URL.
Please check if SAML configuration is assigned to the right theme and users actually hit theme's login page:
For example, in this case, users should open https://%FQDN%/OKTA in their browsers to authenticate using SAML.
You should configure SAML to use the <Default> theme if you'd like to use just https://%FQDN/