i mostly use the Wilhelm method which has you use 2 (1mm) washers on the top of the mobo, i don't use any under and that's just my preference. Also I line the edges of the mobo with a credit card/ phone cardof a thickness of .7mm to ensure even distribution of pressure. I was getting the same problem when I did my first fix and it was because I didn't use the card pcs and because I didn't bake.
Here's the link to the tutorial, this is ver.2 and is more in depth, if you want just use it to understand the "credit-card" placement idea, but i recommend following it to the T, and if you really want the fix to stick reflow the chips first, second link.
Hope this helps and best of luck to you. Follow the directions exactly and you should be golden for a lot longer than the average x-clamp fix. I have repaired quite a few with these methods and have had not problems yet.
Also you can add the pennies, but I don't recommend adding more than 2, this can cause flexing which you don't want.