i can't be choked in jiu jitsu for some reason

we where doing chokes yesterday and the rnc doesn't work for some reason on me same with the mounted one arm guillotine does any one have this problem

Source: http://forums.ufc.production.sparkart.net/showthread.php?t=112038&goto=newpost

Alexander Gustafsson Jaime Gutierrez Dave Gomez Keith Hackney Matt Hamill