Can't enter Tier 2, despite having been there many times.
This has happened once before, but the previous time it cleared up very quickly -
Basically, when I go to the arcade, and click Tier 2 so that I start in Moorcroft Manor, the entry button is greyed out. I've got all 3-star gear, I've been in T2 many times, this is in no way shape or form a question of not qualifying to be there... it just won't let me go for some reason.
Is this a bug? Is there something else going on I'm not aware of? Any help would be greatly appreciated.
So (just to be sure I have this right) -
If I've walked all the way through T1 on a given gate (Say... Silver Pawn), I can go down to Moorcroft Manor for free? But if I want to go down there on Red Bishop, where I haven't done so, I have to pay?
(I didn't realize there was a crown fee really... it just never crossed my mind. Oops!)

That's the gist of it. You don't actually have to go through all the floors of Tier 1 for a gate to negate the crown fee. For example, you might join a party on depth 7 and head to depth 8 (Moorcraft Manor) and get the same results. Energy costs still apply unless you end up starting in a terminal, Moorcraft Manor, or Emberlight.

every gate is separate and each one requires you either pay to descend to either T2 or T3, or fight your way through the tier to get rid of the fee for the next tier (if applicable)
Ahh okay, really appreciate the information! Thanks a bunch everyone, very helpful!
You might not have enough crowns to pay the fee to jump straight to Tier 2. The crown fee only applies to the gates where you never reached Tier 2 by traveling through at least one of its Tier 1 floors.