i used brand like nanny and recently convert to pigeon and alternating between both. i also lost a lot of milk when using nanny previously. but i suspect i tend to put too much in one pack.
try to put lesser in each bag, before you zip up, remove as much air as possible. lay the milkbag flat to freeze, then once frozen the. arrange in standing position if you want.
see if it helps.
pigeon and nanny both uses gamma ray (highest standard of steraliZation)