I experienced that most of the time, buttons not showing up are caused by errors in the template. I think maybe you forget to add some buttons to your elements: Generic template
In my case, an element looks like this and shows up on Messenger:
{
"title": product["name"],
"subtitle": "Product", # workaround for cmdline button interpretation
"image_url": product["imageUrl"],
"buttons": [
{
"type": "postback",
"title": "Add",
"payload": '/user.selected_products',
},
]
}