I spent a bunch of time training my GWP with frozen birds. During the off season, I would grab 5 from the freezer and head to a wildlife area, or an area I hunt, leave the dog in her kennel or tethered where she couldnt see me, and go walk into the brush and toss or stash the birds in a few spots I had marked. Then would go back and let her out. At first she didnt want to dive into the brush, but if I tossed a rock in and told her BACK she would go running around to find a way in. Eventually she learned that sometimes there is totally a bird hiding in the brush, which for her is the greatest thing ever, and now she will aggressively climb through it if she catches a strong scent.
Half the fun of having a gun dog is watching them slowly get better over time. But it does take some time.