I have experience on both ATC and airline side of the radio and I couldn't agree more with people who wrote that pilots are reluctant to call ANY problem regarding fuel. None of them wants to undergo an investigation or do any paperwork. Moreover, from my experience, many ATCOs AND pilots do NOT distinguish between minimum fuel and mayday fuel
What I usually encourage fellow ATCOs is to ask pilots on their total endurance including final reserve. That is sometimes the only way to establish whether the reticent pilot is in fact in an emergency state or not.