recently I took apart my pcg-91111m for cleaning and when put back together I had the same problem.
After taking it apart 3 times to check if I forgot/damaged something I found out that my top RAM slot was malfunctioning although I didn't remove/touch these in the 1st place (which I actually should have before cleaning..).
I found that I had to test each slot seperatly with each RAM card individually.
If this is your problem I recommend buying a single 4gb or 8gb RAM card for the working slot and keeping the defective one empty (since you won't be running much with your 2gb cards..).
I put in a 4gb and this has worked well for me although had I given it more time I would've bought an 8gb.
Also I believe the pcg-91111m motherboard doesn't support more then 8gb RAM so don't bother with a 16gb card (they're also really expensive..).
I found that 4gb cards go for about 20 dollars each and 8gb around 30 each.
Hope this helped, and good luck.
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