WARNING!!! 20-POUNDS OF PRESSURE IS A LOT LESS THAN IT FEELS!!!
If you're having difficulty seating RAM sticks, take the mobo out. Get a (dry, clean) towel and fold it up neat and flat on a table, put the mobo on it, and then install the chips. The towel will provide enough back pressure to keep the board from cracking.
Also, make sure you check your chips and the sockets you're working with. Not all chips fit in all sockets. Be mindful of the notch and orientation. Make sure you touch only the edges, and apply light, even pressure. Sometimes, it just takes a long time for a new RAM slot to "spread" enough to let the RAM in. Above all, don't throw your weight into it and don't "shimmie" the RAM in.