You don't center it, you walk the beam across the resonator for best power in both axis and live with where it lands. Sounds like you need a walk and a optics cleaning.
"Walking" the resonator for peak power.
Once you have it lasing, start with the verticals, detune the output end for a 20% decrease in power. Move the rear mirror in the same direction, see if power increases. If it does, keep slowly detuning the front and follow up by moving the rear until you see a decrease again. If power decreased when you first moved the rear, or if lasing ceases, go back the other way and look for a new peak. You then do the horizontal in the same way, then return to the vertical adjustments, going back and forth and back and forth in smaller steps each time. Your trying to find the path of max gain through the tube, and often due to slight mechanical distortions during manufacturing, it is not exactly on center.
BTW the front panel power meter is most likely not calibrated for "white" light but for the 514 or 647 line.
Steve