Splitting the forum from the main site's domain name, inventorspot.com/forum -> inventorspotforum.com, seems to have something to do with it. We split the username registration systems since we were having trouble maintaining the integration between the two. At the same we thought it would be a good idea to give the forum its own domain name. Without inventorspot.com's domain rating, we started over as a "new" site in the search indexes. We have redirected all of the old links, but it takes some time to recover. Hopefully all of the traffic will come back.
All I can say is, tell all your friends about us, and participate!