After the Highbridge scene (back in Bevelle after Yunalesca when Grand Maester Mika bricks himself about losing her), there is no chance of seeing Mika again in FFX. By the sound of it you're trying to open the Baaj temple, and think you missed something in Bevelle, correct? Well, good news, its impossible to complete the Bevelle temple without getting the destruction sphere that helps unlock Baaj (and it is impossible to get back into the Bevelle temple after you get Bahamut). You've more than likely missed another temple, though which, I wouldn't guess.

'Course, I could've completely misunderstood you