There could be a couple of reasons. First of all, know that you can only have two concurrent sessions with Ameritrade. Option Stalker Pro will count as one and the other could be TOS or mobile, but not both.
The other issue could be that you have two accounts with Ameritrade associated with the same login. If this is the case, you need to ask Ameritrade to separate the logins so that you can use the API. This is how they configured the API.