I have tried to setup the integration few months back in a UAT and seems to have went fine for us
Take a look at the below link for mapping
Alarm Fields, REST examples and Attribute mapping - CA Spectrum - 10.2 and 10.2.1 - CA Technologies Documentation
Alternatively you can further troubleshoot by accessing the below link
http://<OC_HOST>:<OC_PORT>/ca-nim-sm/ui/login.jsp
Login as nimadmin/nimadmin
Click on customizations and select ServiceNow from drop down, select appropriate CI type and you should be able to see the related mappings between Spectrum and SNOW here via api
Note: Support do not recommend modifying anything after logging in using the above link, so please be careful