I have a 2-year old Yorkie that I am having problems with potty training. I read in a few Yorkie books that we should start off by placing potty pads in various places around the house, then slowly transition into the outdoors. We removed the potty pads slowly after we monitored his usual times for going to the bathroom, then put him outside during those times. This worked for a while and he wouldn't poop inside. Unfortunately, he now has gone back to his old ways, but he poops where the potty pads used to be. How can I effectively communicate to him that there isn't a potty pad there, so he can't poop there?
The fact that he's doing his business on different substrates (on the pads, on the floor inside and probably on the grass outside) seems indeed to indicate a lack of potty training, especially for a 2-year old dog.
I would not be concerned by the fact that he's pooping on the floor where the pads used to be. That's a good sign: he remembers where you wanted him to poop.
When you let him go outside, try to bring him on the same substrate every time otherwise he might be confused.
I would just go back to puppy potty training basics: prevention (the most important part - that means put him outside often), no yelling if he poops inside, praise him (even if the neighbours think you're crazy) when he poops outside. If you catch him inside stop/distract him, bring him outside and praise when he's done. If he's on leash outside, give him time to play/pull a bit before continuing you're walk, that will serve as a reward.
When you take him out, take a potty pad as well. Let him do his business on the outdoor potty pad. Start making the potty pad smaller, first cut it in half and just use half of it. Then keep using smaller and smaller pieces. Hopefully at some point he will transition from pad to grass as the target, but if not change from a potty pad to a tissue or some other biodegradable product.
If you place the potty pad in different areas you should have better results in the transition from pad to grass, though if you always place it in the same area it could make clean up easier. You will need to periodically adjust your training approach based on your dogs responses.