Modern boards use plastic stand-offs to which the board simply snaps into place or is fastened to by ties. Older PC casings use brass ones that need paper or cardboard washers for the task. There may also be an issue with the alignment of the board itself (there was an issue with Gigabyte boards sometime back that had the whole board sagging in one place of the other which prevented the memory modules from seating properly. Thanks to answersthatwork.com for the details ) Use plastic tie-downs where a plastic tie is used to pull the board into alignment to solve this type of problems.