Do you have a dog that is fantastic at so many different behaviours in the comfort of your living room, but then if you go outside, or to a new location it's like he doesn't know what you're asking him to do?
There are several reasons why your dog might not be responding to cues you have already trained, (we won't go into them all now, but one day we will) and one of the reasons might be because he hasn't ever been properly taught in other more difficult scenarios. Dog's aren't as great at generalising as what people are, (sometimes people aren't great at it either!) and this means that he needs a bit more training with more distractions, remember when training your dog in a new location, make it easy for him to succeed.
The thing to remember when dog training are the three Ds, distance, duration and distraction, once your dog is reliably performing the behaviour you have asked with a variety of distractions, at a duration that ends with a release word* and distance between you and your dog, then it is safe to say this cue has been proofed. Don't rush a head, take it slow, enjoy dog training! Remember the tortoise did not win the race with speed, he took his time and relaxed. Training your dog should be the same, enjoy it and don't put any pressure on your canine or yourself.
*A release word is a cue to give to the dog to say they are ok to be released from that position, whether it is a down, stay, sit, stand etc.