If you have a flexible supply, make sure you aren't using Tefon Tape.
A flex supply will have it's own seals at the ends, and something like tape will prevent you from threading up the 7/8" nut to the tank fill valve.
That, and it helps if you are left handed. And don't drink and then try to connect the flex line. You're going to need your wits about you, oh for sure.