If the oxygen absorbers are not hard, they are in good condition and working. All the following factors may contribute for you to not see the shrinkage of the bag:  - how much void space is between stored food - how much headspace is at the top of the bag - whether there's a hole in the bag - whether the seal is good - if sufficient size of O2 absorbers is used You may often see the vacuum effect, but if you don't that does not mean the oxygen absorber didn't work. As you know, only 21% of air is oxygen.   Oxygen absorbers only absorb oxygen in the air which is 21% of air. That means roughly 1/5 of the air volume will be removed, but 4/5 of the air remains no matter how many oxygen absorbers are used.  Most of the air is nitrogen, so even oxygen absorbers have absorbed oxygen, you may not see a big reduction in the size of the bag.   Just because you don't see the shrinking of the bag does not mean the oxygen absorbers are not working. After the mylar bag has been sealed shut, it may take a couple of days or even a week to see a slight vacuum seal inside a bag.  Therefore, it’s a good idea to get rid of as much air as possible when you seal the mylar bag.  Also, how tightly the food is packed can affect the time you see the reduction of residual air.  For example, when storing beans, it will take less time as there is much room in between the individual granules versus flour/milk powder which is highly dense.  If you don’t see a slight reduction in the residual air, it does not mean it’s not working as long as you have sealed your mylar bag well.  However, if you are in doubt, you can do it again.  Just create a little slit in the bag and add a new oxygen absorber and heat seal that part.  You don’t have to remove the previous oxygen absorber.