Asus F5RL RAM memory present but not recognised

[Mecha]

Hi there

I have been reading around and, although there is a variety of posts regarding this general issue, none addresses my particular problem.
I have recently upgraded the 1gb RAM coming from factory to 2gb by a new 1gb module. The maximum memory that can be installed is 2gb Asus F5RL Laptop Memory Ram Upgrade :: Asus F5RL Ram Upgrade :: Offtek.co.uk, the Memory Superstore
I have tried the two modules in all positions (each on their own and combined) to no avail: the system always recognises 1gb (895 usable) but never 2gb. Furthermore, following the recommendations found here http://www.sevenforums.com/hardware-devices/184390-asus-f5rl-memory-problem.html I have set the maximum amount of memory to "all available" and have run the RAMmon test, which shows both memories present but only 1 gb available.

I have also upgraded BIOS and applied all tips I have come across as well as trying with other OS, but it still escapes me why the system cannot use the 2gb when it seems there is not any hardware problem.
Any idea where the solution might lie?

Thanks very much for your help.

 

[mrdemon]

The 2 sticks may not be compatible!

 

[Mecha]

I did not even know that that could be possible at all.
How come that two modules with the exact same characteristics can be incompatible? In any case, does anyone know how I can check if that is the problem?