Registration Notification Doesnt Work

I setup a registration notification URl like this

api.emailcampaigns.net/2/REST/?method=List_Subscribe&key=abcd123&ListId=147&email=%%user_email%%

When I use the correct key above in place of abcd123 and go directly to this URL with an actual email address in place of %%user_email%%

it updates my backend system OK.

But i cannot get the plugin to work automatically- I have this in place-
api.emailcampaigns.net/2/REST/?method=List_Subscribe&key=abcd123&ListId=147&email=%%user_email%%

and I registered several new test users and none of them make it to my backend system. It doesnt seem to fire off as it should