SMS limits
Currently per user there is a limit of 6 SMS login codes that can be sent per hour. Once this limit has been exceeded you will need to wait 60 minutes before logging into your account again. This is to comply with security regulations.

Receiving your SMS
Your unique code being sent to your mobile device is reliant on telephone signal. You cannot receive the code without this. If you are unable to receive the SMS code, you can use one of the backup codes you have downloaded.