What can I do to break this habit?
In a nutshell this means you’re starting to lose control of the dog. And if he’s ignoring you now it won’t be long before he’s wandering here, there and everywhere he pleases.
Instead of letting him walk free you need to put him back on the slip lead and insist he walks strictly to heel i.e with his front shoulder almost touching your knee.
If you’re right handed he needs to walk on your left side and vice versa if you are a left hander – this way when you’re walking with a gun on your arm the barrels won’t keep knocking against the dog’s head.
Walking the dog to heel on a lead while picking up isn’t going to interfere with its ability to mark pricked birds but it does mean you stay in charge of the situation.
Every time the dog now tries to stop and sniff, or to pull to one side, say “leave” and command him to “heel” with a stern voice and a tug on the lead.
To reinforce the order you should also pat your left leg to encourage the dog in close.
If he continues to push his luck, get his total attention by reaching down and giving his ear a quick tweak with the “heel” command, to remind him just who’s boss in this relationship.
Until you get him ignoring smells and temptations and walking to heel again properly, you should avoid carrying the gun because it’s not possible to concentrate on the dog, and your shooting, at the same time.