Try this Rob:
1: Make your Loop; then Drag and Drop from the Track Title ‘While It Is In The Loop’ to the Sample Deck
2: The AutoGain Feature in Traktor is so that you can ‘Set The Volume Of 2/4 Decks’ and then Leave Them All Night and they will Stay The Same – the AutoGain doesn’t just Set The Volumes To Be The Same Automatically
In Preferences you can Turn On AutoGain; and then Every Track that you bring in will be ‘Analysed’ for Volume as well as BPM; and will Adjust the Volumes to be the Same;
You can Check This (and Make Sure it has Done it) by Right/Control Click above the Traktor Browser; and in the ‘Grey Area’ Choose to ‘Show analysed Column’; this Column will show you All the AutoGain Values
I hope that Helps Some 🙂