I could be off on this, since I haven't been watching the time too closely, but I feel like this is 3 days in a row that the inventory has suddenly increased around mid-morning. Today it was sudden - was trying to order and it had been the same choices pretty much all morning, got an error message, refreshed, and there were 9 more bands total, with choices changing on more than that. Thinking about it, yesterday when I refreshed not long after having refreshed a few times throughout the morning with little to no changers, there were suddenly big changes. I think Thursday was like that as well, but can't remember timing. It might be worth watching throughout the mid-to-late morning to see if this is a pattern that holds up or was just a coincidence.
Edit: Perhaps it worked for me today because I was on at the moment the inventory changed and completed my order within a minute or two afterwards.