t841a mechanical thermostat,aux heat not coming on,at defrost cycles(on heat)will come on manually by turning up tstat

I find my aux heat not coming on when temp inside reaches the level to activate aux heat(usually when heatpump goes into defrost mode or outside temp below 30 degrees,I often clean inside thermostat for dust,may have recoiled or compressed winding temp spring? my question is there an adjustment to move the lower mercury bulb(which is the aux heat more level with the temp setting merc bulb on top,or how can i bend to have lower bulb more level thanks sam

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