weird memory problem?

I find this problem slightly weird, and after researching I cant for the life of me think what the problem is... here goes

I upgraded the memory in my macbook 13" 2ghz a few months ago with some ram I bought off play.com, this week I went and bought the same stick of ram with the intention of upgrading it to 4gig, however the macbook doesn't like the new ram alongside the upgrader 2 gig,

for example, I can still run my original 512mb and the 2gig old stick, I can run the new 2 gig stick on its own, or with the old 512mb, but I cannot get the 2x 2gig sticks to run together, it gives me the 3 bleeps?

they don't look identical (even though I thought they should be) but its clear they both work, but why not together?

any suggestions?

 

I can only put it down to the fact that they are clearly different memory sticks (despite they're supposed to be the same) but they shaped differently - is the problem, as my system (according to crucial.com) can hold upto 4gb

Maximum Memory Capacity: 4096MB
Currently Installed Memory: 2.5GB
Available Memory Slots: 0
Number of Banks: 2
Dual Channel Support: Yes
CPU Manufacturer:
CPU Family: Intel Core 2 Duo 2 GHz
CPU Speed: 2000 MHz
Although the memory can be installed one module at a time, the best performance comes from using matched pairs of modules


maybe its because they're not identical, but then why would it run with the old 512mb stick alongside it? confused

 

Well what i've done is explain the play.com the problem, and their shipping me out the same piece of ram, so im going to try the two new 2gig sticks and see if that works, if it does then ill send them back the old 2 gig one back

ill keep you guys posted