Pasteurizing problems

Hey guys, I am new to growing mushrooms at home but thought it sounded fun to try. Right now I am growing oyster mushrooms in straw and things seem to be going good with those, my logs are almost fully covered in mycellium, but I want to try my hand at growing lions mane. Now my problem is, after watching I dont know how many videos is pasteurizing grain in the pressure cooker. To save money I thought I would try it myself. I have gone thru all the steps of soaking and then boiling and then putting the grain in bags and loading the pressure cooker and packing towels around the bags to take up the extra space but after the 90 min of pasteurizing and letting cool overnight, my bags are expanded and not tight around the grain like I have seen in videos.  Any ideas of what im doing wrong becouse I have to do the same thing when i make my sawdust bags for the lions mane,  PLEASE HELP....

When you say expanded, do you mean they are puffed up like a balloon or just not sucked in?
Not being sucked in could be a few things, the first would be that your closure is not airtight a second could be that there is either a small hole in the bag or the bags are to thin plastic and air can over time go through them.

Another avenue would be are you filling them hot? If you are filling them hot then I would say one of the above ideas is happening, if you are filling them cold then that will be the issue. once you close them the amount of air in the bag will not change since warm air takes up more space than cold you want to make sure it's warm air you fill the bags with originally.

Why the towels? I’ve never heard of that before.

What kind of bags are you using? Do they have a filter patch? I never cook grains. I soak over night in room temperture water (8-16 hours), drain water thru colander or sieve. Roll grain in a towel to pick up extra moisture (sometimes not always). Fill bags or jars 3/4 full. Add a bit of gypsum to each bag to prevent clumping. Shake jar or roll bag to coat grain with gypsum then load in pressure cooker. I have never heard of packing towels inside to fill up space. Space is a good thing. Make sure to have something ito keep the bags off the bottom of the PC.  My 2 cents.