I doubt this is anything to do with your TalkTalk connection, if the same stick is causing issues when you swap them over. If one is connecting fine, then there would not seem to be anything wrong with your TalkTalk connection.
If you mean it's the same connection on your TV that a stick is not working on, then it's probably the TV at fault and not your TalkTalk connection.
Either way I suggest you reach out to the Now TV community who also have a very good forum to see if they have a solution.
have you tried resetting the Now TV stick, and then going through the set up procedure again? This may help.
hope you find a solution.
Can I ask why you think it's your TalkTalk account causing issues?
On the Now TV site and YouView forum there seems to be many people complaining that apps on various devices are causing display issues.
The fact one of your sticks is working tends to suggest your connection via TalkTalk is ok, and the other stick is at fault.
If you can pinpoint a TalkTalk issue staff here will look into further.
@fre55die as a Now user, any thoughts?
As @fr8ys says, Whilst I personally hate the NOW sticks as I found them unreliable. It sounds like a faulty NOW TV Stick problem as one works perfectly and the other doesn't for all channels.
How old are they ? Do you get any warnings from stick and are they individually registered on your NOW TV account as you are limited to 6 devices, have you exceeded 6 devices on the account.?