ram fault

For the last 1 month, I have been noticing that , although my PC has 2 sticks of 8GB RAM = 16GB, but all of a sudden it is only showing 1 RAM. Looks like 1 of 4(in task manager) in the picture. If I restart my PC, it becomes fine again. Then 2 of 4 (in task manager) show. That means I am getting...