Not receiving Push Notification to Windows Phone 8

I have successfully registered a Windows Phone 8 device, but if I sent a message through my app or the API it does not send anything to the phone. I've also registered other non windows devices and they are receiving the PN correctly. If I send a message to a list of devices using the API like this:

{ "request":{ "application":"XXXX-XXXXX", "auth":"XXXXXUb", "notifications":[{ "send_date":"now", "content":"XXXXXXX", "data":{ "type": "Notification", "document": { "name":"Name", "lastname":"Lastname" } }, "devices":["APA91XXXXXXXXXXXXXX","APA91XXXXXXXXXXXXXX","http://a.notify.live.net/u/1/hk2/H2QAAACCT5eH_XXXXXXXXXXXXXXXX"] }] } }

I receive this kind of answer

{ "status_code":200, "status_message":"OK", "response":{ "Messages":["0F6F-D208B55A-F68C0F83"], "UnknownDevices":{ "0F6F-D208B55A-F68C0F83": ["APA91XXXXXXXXXXXXXX","APA91XXXXXXXXXXXXXX"] } } }

2 of the devices were correctly sent, but not the Windows Phone one. But if I send a message through "Send Push" in my app, I receive the message in my phone.

Does anyone have an idea that what could be happening?

Thanks