iMac 2.4ghz 20" - not recognizing Memory

I had purchased an iMac desktop, the 2.4ghz 20" screen 250 HD version, and tried to upgrade the Memory capacity from the factory installed 1 gigabyte to 4 gygabytes. I did some research prior to the upgrade, and verified that this model's memory could be upgraded to 4 GB, but after taking out the original 1 GB memory card and installing 2x 2 GB cards, the system would only recognize 2 GBs of memory, NOT the total 4 GB's.
I bought the 2x 2GB memory cards from Amazon, and these are the fully compatible DDR2 cards for iMacs and Macbooks for my 2.4g 20" iMac model... so I'm wondering why it only recognizes 2 GBs in the system memory when 4 GBs were properly installed? Any ideas??

G'day and welcome to the forums.

Sounds like inappropriate modules. Use this link to OWC Mac Specialists to check out your memory requirements and are all the modules Mac compatible as there are DDR2 modules for PC's and Macs, some being high density and Macs require low density memory. Are they all matched?

Apple iMac Memory Upgrades and Video Installation Guides - Get the right memory for your G3, G4, G5, or Intel iMac Model

It's also possible one of the chips is just not seated all the way or is bad.

You need to test, by pulling one at a time and try the other slot with each. If you have a bad stick, get a replacement... It does happen.

got it working now...

thanks for the advice. turns out was just a simple matter of fully seating both memory cards in the slots, got to firmly press into them to get them seated properly. Just rebooted and the system now recognizes the two (2) full Memory cards, 4 GB of system memory total now!