Leaking from underneath the locking ring of the AVENT Bottle most often occurs when the 'skirt' in the base of the AVENT teat is not in the correct position. Try the following steps:
Pull the teat through the locking ring until it clicks into place. Then put the dome cap firmly on to the locking ring. Screw the assembled top section onto the bottle.
Check that the top section has not been over-tightened. This can distort or trap the anti-colic skirt which will not allow the proper operation of the one-way valve.
Wet the base of the teat before pulling the teat through the locking ring. This will ensure that the skirt will slip easily into place inside the neck of the bottle and form a seal.
Ensure formula powder is not present on the inside top ¼” of the bottle.
Avoid over tightening, of the top section, screw ring, teat & dome cover onto the bottle base.