I train my dogs to "hunt dead", with the bumpers hidden in the cover at intervals, with a stacatto whistle (sounds kinda like a feeding chuckle with a whistle). That way, when on a blind (or a difficult mark for that matter), when they are near the fall in heavy cover, I can give them an assist when they are in the vacinity. Pretty easy to condition and helpful in many hunting situations.
In training, with dog at heel and nearing the planted bumper I give a sit whistle, a "dead bird" command, a "ok hunt 'im up" with your line hand down, with the stacatto whistle slowing down when she gets near the 'bird', then a quick "come in (tweet-tweet)whistle" when she picks it up. Doesn't take pup long to get the hang of it. Gradually put more distance between the sit and the hidden bumper. Later, put more distance between you & dog and the hidden bumper (a blind) and using a remote sit, cast and the "hunt dead/dead bird" whistle. Just another idea you may or may not want to try. I find it useful.