A few of my friends are using Avent milk bags and none had experienced any leakages. If it happened to all the bags that you've purchased, I would suggest that you contact Philips Avent to see if they can offer any solutions.
http://www.philips.com.sg/c-w/support-home/support-contact-page.html
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Did u drop the fbm on floor or knock it anywhere ?
Mine doesn't leak.
Leakage only happens after I knock it somewhere or drops to the floor