Lael is almost 3.5 months, and just recently began to lay down for me very well. She used to stand right back up and walk off from frustration.
So here's what I did: if I stopped praising her, or told her to "lay down" again after she got up, she would NOT do it. She just didn't understand. So if she even slightly laid down I would give her food and praise her (even if she had popped back up). ...over and over and over... She eventually liked to be down because she was getting so much love and praise for this action. She also was no longer confused as to what I wanted her to do. As SOON as she touched her chest to the floor came the food and praises. Then...(after some days of laying down more and more) once she laid down I would say "stay..." and bring the food up, up, up and over her head. And she got it! She stayed laying down while I walked around her with the treat over her head. Then I gave it to her, of course, and made the biggest fuss ever!
Hope this might help.
Cat & Lael