Manual valve body 727 - 3rd gear won't engage under full throttle

Hi Gang! Having some issue getting my 727 to shift into 3rd gear under full throttle.
Some background here- The trans has a reverse pattern, manual shift valve body in it. The 1-2 shift if great, but when I go to shift into 3rd under full throttle, the trans will not shift. If I let off the gas some, it will shift, and then feels great once I get back in it. Under light load, it will shift. Obviously this is not ideal for a drag racing application. Any ideas why this would occur? Might I have a sticky servo, or band out of adjustment? Thanks in advance for any responses.