For PC RAM slot leave empty for a long time, there could be oxidization and make the poor connection, try clean the RAM slot use a soft cloth or put some alcohol on a cotton stick then clean the RAM slot and wait 5 minutes for it to dry out before installing the new RAM.
The Crucial scan doesn't always work. Reported my Acer notebook as having 0 (zero) RAM fitted witha a maximum of 0 allowable. Support was first class, I emailed them and fitted two 1 gb modules in dual channel mode. Well worth it, great improvement. Great service too.