[Marilize]

Most yorkies don't like to lie down, especially on wet grass. Try using an insect repellent to make sure that the bugs don't get on him and you have peace of mind.
You need to condition him to perform his behaviors anywhere, so I would try moving outside for his training. Try to get him to perform on concrete, wooden decking, tiles, anything he wouldn't normally be training on. Slowly work your way up to the grass.
When you finally feel he is ready for the grass and he won't put his body down, try putting a blanket on the ground for him to lie down on. Don't keep it down for too long, just use it a couple of times to get him to perform his tricks perfectly. Make a big fuss of him if he gets it right. As soon as he gets it right, give it one more go on the blanket and then remove it.
If he still doesn't perform well on the grass, don't treat him for half-assed attempts. He knows how to do it right, so he has to. Don't let up until he does it perfectly and no more praise and treats for poorly executed tricks.

Sorry, I can't help with the equipment issue, but I am sure someone here will be able to help. Good luck.