They don't know why they are outside is my guess. Usually dogs like to please their owners...just being outside does not mean they know what is expected. It is best to take them to the same spot every day and not dally around trying to get them to go. If they go, praise them a lot...if they don't go, bring them back in but don't let them out of your site. Put a leash on them if you have to until they get the hang of it. Then take them out..do it until they go..praise the daylights out of them. It takes patience and a lot of repetition.
Maybe you can find some ideas here:
Free House Training for Puppies and Adult Dogs - No More Accidents Starting Today!
Maybe the trainer will help. Good luck!