Forums weird indexing

I have this problem for a month now and a friend of mine started noticing it at his forums as well. After months of very good indexing and perfect SERPs, google suddenly slowed down at indexing the forums pages but kept indexing the archive! Also, when I make a search now, it shows the archive results only. At the same time, the forums have sitelinks and they appear first when you search for its name.

Is there a known reason for this? Did anyone notice such thing?

Thank you.

 

i don't know any reason...but you can ask your question direct to google and maybe they answer

 

I used the reconsideration request but they don't reply. Do you know a better way?
Thank you.