If after aligning everything and trying to push one side into the socket, then the other, and having no joy, consult somebody with experience. Sometimes a second pair of eyes is all you need to spot an oversight. Provided you have the right RAM, properly oriented, as LudazTV suggests, you may just not be applying enough force to cause the RAM to fully lever into the slot and cause the end clip to clamp down.
Normally, I prefer to install RAM modules into a motherboard before installation into the chassis, but if the motherboard is already installed, I find it easier to put a bit of pressure on the end clip while gently rocking the module 10° (give or take) both ways along the socket axis while applying pressure to the top edge of the...