DIAGNOSIS The tamper handle has swollen or warped. This is common with older rubber-grip tampers exposed to heat and oils. The lid hole dimension is fixed.
FIX Clean the tamper handle thoroughly with soap and water to remove any oily residue. Dry it completely. Lightly sand the rubber grip with fine-grit sandpaper (220 grit) until it fits smoothly through the lid without binding. Wipe away all dust.
IF THAT DIDN'T WORK The lid itself may have deformed. Check if other tampers or objects also stick. If so, the lid needs replacement. Order a new lid (part # 14460).
HEADS UP Never force the tamper through the lid while the blender is running. This can crack the lid or cause the tamper to be grabbed by the blade. Only use the tamper on variable speed 1-4, not on high.