I'm not sure exactly how you're shifting down, or if you're trying to go past several gears at one time. I can do what I think you're describing on my Volutia.
However, regarding the Thunderbird, page 44 of my owner's handbook, 2nd column, top, in bold print, says:
"The gear shift mechanism is the 'positive stop' type. This means that, for each movement of the gear shift pedal/lever, you can only select each gear, one after the other, in ascending or decending order."
Perhaps you're talking about something completely different, but thought I'd mention it just in case.
"Do what you can, with what you have, where you are." - Franklin Delano Roosevelt