inspiron 1720 will take one 2GB stick but not a second

i've done the research and found that i should be able to install 4 GB of ram.....after installing a pair of 2GB DDR2's, the laptop would not even start up. i then tried it with just one 2GB stick (in the top bay..under the keyboard) and one of the original 1GB sticks in the lower bay and it started right up...and recognized the new memory. its working with 3GB in it...but why can't i add 4?

thanks in advance

here is what i have:

Dell inspiron 1720

windows 7 ultimate sp1; bios ver A09

intel core 2 duo t5250 @ 1.5 ghz