“Combo Master” must be face-up on the field when the chain occurs in order to be able to attack twice. If a chain occurs, and then this monster is flipped face-down and then face-up again, it cannot attack twice. (Unless a new chain occured after it was flipped face-up.)
If a chain occurred, then “Skill Drain” was activated, but then “Skill Drain” was destroyed, this monster can attack twice. (This monster’s perception that a chain occurred is not reset.)