Hi and welcome.
It is quite possible that the different memory modules are just not compatible. I know what you said about the same make and model etc but sometimes even that doesn't guarantee that they willl work together. This is why it is always safer to buy a matched kit. These are actually tested electrically for compatibility with each other.
What are the modukles make and model number anyway ?
It would be worth running Memtest as follows though just to prove that the modules are all ok.
Please follow these instructions
exactly.
I would suggest that you download and run the latest version of Memtest86+ to check your RAM first.
Memtest86+
http://www.memtest.org/Insert
one stick of memory in
slot 1 and run Memtest on it for at least
10 complete loops/cycles and if there are no errors then swap it over with the next one and continue untill you have checked all modules.
If you have any errors the module is faulty.
If you have any faulty modules you will have to return the whole kit as they are matched.
Post back when you have done that with the results.