It's been a little while since I had one of those apart but IIRC there's a little valve, operated by a wavy spring washer, that opens at topout, equalizing the positive and negative spring pressure. My guess is that isn't working right, and the negative pressure is too high. Just a spring side rebuild/relube should fix it.
My boxxer did that at whistler last year. The fix was what HAB essentially laid out. The schrader valve had gotten grease in it and wasn't equalizing. So go light on the slick honey or whatever you're using.
I did the rebuild in a parking garage so it's not too difficult.
Those guys are both sorta wrong, the 2 step air cart is completely different than the Boxxer air spring.
D/L the instructions from the Sram website, but essentially you just need to disassemble the 2-step piston and main air spring, and put it back together. A very simplified look at the issue is that there is a plastic piston on top of the main air spring that moves up and down forcing oil through the top cap every time the fork compresses. When you lock the fork down, you are closing off the return flow. So you compress the fork, oil flows through the top cap and out of the 2-step, doesn't return and holds the fork down. When you switch back to full travel, the valve opens and the oil flows back and allows the fork to reach full extension.
From time to time, the piston either seizes, or starts leaking and you lose travel. I have to rebuild mine once every few months. It's part of my regular service schedules. I usually rebuild my Boxxer and Lyric air springs at the same time.
Take the top cap off, dump out the oil from the 2-step system, free the plastic 2-step piston, dump excess oil out of the main air spring, and re-install the 2-step piston, add the correct oil amount, and bleed the 2-step cap via the bleed port.
Total time, roughly 30 min. Oil will get everywhere, so make sure you do it off the bike and away from your brakes.
EDIT: Rockshox went away from the 2-Step and is now just doing the "Dual Position Air" on the Lyric which is a different system, and I am not familiar with it.
its a very common problem with the 2-step.
ideal solution. throw that rockshox piece of **** away.
next to ideal solution. let all the air off the air-spring. then take off the 2-step knob with a 2 or 2.5mm allen. when a 24mm socket wrench unscrew the cap of the stanchion a little, until you let all the air on top of the spring out. screw everything back up, and pump it again.
you should get your 20mm of travel back again.
i had to do this every 2-3 rides with my totem. got so profficient i could do it under 3 minutes.