First: You just released Clan mechs. So many of those who bought the clan mechs are now eager to level them up and enjoy their new aquisitions. Asking them to leave those behind for even a couple of matches really isn't going to work without an incentive.
Second: Incentives. You need to give players a reason to hop onto PTS. Why don't you unlock Clan mechs for everyone on the PTS? (This is not my idea, I saw it posted somewhere else on the forums) I did not buy the Clan packs, but I'm definitely eager to test drive a clan mech. That would entice me (and many of my friends) to load up the PTS to get a chance to drive the Clan mechs and see how they feel.
This would accomplish many things. Those who feel the Clan mechs are "OP" will be able to understand how Clan mechs really are by piloting them first hand. They could see which Clan mech they like most and this will certainly push a few people over the edge into purchasing a Clan pack because they don't want to wait another 3 months to get another taste of Clan mechs.
Additionally, I'd consider offering people 1 day of Premium time if they commit at least 2 hours to testing on the PTS when you open it up. This would probably get more people involved, even if they own Clan mechs already.
Third: This is speculation on my part, but I think a lot of the guys who were waiting around for the Group Sizes of 5-11 got fed up and left a few months back. I'm not sure what you could do to entice them to return to the game, but this shows the importance of NOT burning bridges with any segment of your community. I may be wrong about these assumptions though, and I'll let some of these "mid-size group" evangelists offer their own thoughts and opinions on the matter for themselves.
Fourth: Weekends are better. Like a lot of people in the world, I work. Trying to fit the PTS into my schedule is challenging enough on the weekends, but doing it during the work week almost garantees low numbers. You saw it yourself with the Clan tests, didn't you? The first one was huge, but wasn't the weekend one even bigger!? I don't know if you're worried about strain on the servers or what, but if you truly want a good test sample of users, you have to work around our schedules.
That's all I had to say. I want to help you guys out, but you need to help us out, too. Free mechbays and premium time is awesome (and much appreciated), but instead of giving them away for free, make it a reward as part of civic participation for the greater good of the game.
Edited by ShadowVFX, 30 June 2014 - 10:28 AM.