GTM & Script using 1st party or 3rd party cookies
First of all, check, if you have set the correct postback in your campaign settings.
If the postback is correct and is on place, check if the click id data is received from the traffic source: go to Logs -> Clicks -> check ref id column and if there is any data available there.
If there is no data, check if you receive it from your traffic source -> your tracking link should contain traffic source macro={ref_id}.