Installed memory not available

I recently installed 2 new memory cards in my Laptop(intel i5 processor). The cards are from different manufacturers, also one card is 2gb and the seconde one is 4gb. Now when i start my computer, and go to 'Control Panel\System and Security\System', one time i see Installed memory : 6gb, but other time installed memory is Not available. Why is this like this and how to fix this?

If the installed memory should not be available, your pc should not be able to start.
I suspect there is some timeout while reading the available/total memory. Maybe a heavy program is running in background ?
U can check if they are both of the same clock-speed, check installing first only with the one and later with only the other, start and check if in this case the problem still there.