The decline of the activity of the forums are to be expected since the forums deals with the topic it was mainly created for. Playtonic Games, and their games (In this case Yooka Laylee).
When the game is released, the activity will spike upwards. So will also happen for shorter periods of times when news are released (and I mean real news, not tweets).
Also, due to the "secrecy" nature of the team members. (Much like how Rare behaves), it provides even less discussion material.
You must remember that the forums are not like a IRC channel or a chat, and that Playtonic is a very small company at the moment (and will likely remain so, unless it grows 10 times the size).
I used to be in a community that hosted serves for several "games", this community used to be very active. Why it declined is not connected to this forums situation. But it had a high activity because there was material, games and news to be discussed.
Besides, the internet is currently over saturated with forums that deals with gaming, and specific games. Most of them are already strongly established and people who have their "home" there are not very likely to just up and move position to a new forum just like that.
It certainly does not help that these Forums are severely underdeveloped. It badly needs a face lift and some restructuring.
So, what I think will raise the activity is this, in no particular order:
*News, and not just once a month or even longer, Perhaps a Bi-weekly news letter (every 2 weeks).
*Productive Members who produce content.
*The actual release of the game (the Toybox release will raise the activity a bit, but far from the levels of the actual game release).
A serious face lift of the forums, including new relevant categories. Gathering all off-topic under a single sub forum with no categories is incredibly lazy and counter-productive.
Also. It was very nice to see the Playtonic tweet mention the forums. But we need a bit more.
A passing mention now and then should suffice, any more and it just becomes a annoying pattern that will be treated like any advert, ignoring it.