Oliver,

If it doesn't require any lift, you need to remove the four hex-drive bolts that hold the shifter onto the cover extension and have a look in there. I can't picture it exactly from memory, but there's a plate in there that's supposed to block the shifter moving left unless it's lifted. Could be bent, or worn...

After you get that working, you can adjust the spring tension -- how hard the lever is to lift -- by removing the shift knob and turning the long spring in or out.