Properly sealing the mason jars , can actually be tricky with some powders , or when working with hot jars
First thing is to always use a new lid or invest in the reusable taters , If your lid is new out of the slieve just inspect the rubber seal to ensure it is CLEAN , If you must reuse lids clean them well with soapy water and dry well with something that is not going to leave behind fibers on the rubber, If you have reused the same lid many times a quick wipe of alchool on the rubber can help, just be sure to let it completly dry first
When storing dry goods in a jar alone or with a dessicant , You will not obtain the vaccume effect like the store bought jars, They are filled and sealed with hot contents , after the jar cools the contents shrinks (heat makes everything but water expand) and builds a vacumme
When using a O2 assorber you will have a vaccume effect (if done right) , First o2 assorbers require a small amount of water to work so the mushrooms must not be 100% brittle cracker dry , 98% cracker dry is just right, Since you need the small amount of water no dessicant is used with the O2 asorber .
Simmply fill your jar about half way open you package of O2 asorbers and drop the correct amount in, close the package of asorbers, fill the rest of the jar leaving around 1/4'' space between the lid and fill, gently place the lid on the jar without getting anything on the rubber , and lightly tighten the lid down (you dont need to over tighten it , in fact that can cause leaks)
Put the jars away for about 24 hours, after this amount of time you should notice a vaccume has formed sucking the lid down. If this has happened just store in the dark at a stable but cool temp.
If you have not gotten a vacume seal the lid likely did not seal or the mushrooms were likely to dry or O2 asorbers do go bad if not stored properly