It can also be effective to light the action board from below.
Hold a BBQ skewer in your dominant hand and fit the action board in as far as it will go.
When you find a button that doesn't fit into a hole, insert the skewer into it and poke it gently to fit it into the hole.
Rather than fitting them horizontally, it's better to fit each row diagonally starting from the back row.
When you finish fitting each row, the tops of the buttons will be a little stuck in, like the first problem. This is because if you fit too many buttons in, it will be difficult to fit the next row of buttons in.
Stagi/Bastari improvement
https://concertina.pinefield.asia/improvement-concertina-beginners-and-self-taught/