Join Date: Jun 2011
Location: Sunnyvale, California
Bikes: Bridgestone RB-1, 600, T700, MB-6 w/ Dirt Drops, MB-Zip, Bianchi Limited, Nashbar Hounder
Often, if the plastic/rubber thumb tab is bent and you see that it's steel, some firm but gentle prying and work it back so that the rubber cover has clearance and will move freely. But make sure the unit itself is fixed properly and flush with the rest of the assembly. A knock that that can crack the mounting so that it still functions, but is likely to fail soon unless remounted correctly or replaced if the mounting system is broken.
Bending the thumb tab back, however, might be the ticket and if everything else is flush and firmly affixed, then you've saved the cost of heading out to buy new hw.