toshiba can’t find memory up grade

I have a Toshiba Satillait A105-s2021 it say it will hold 2GB of memory. DDR2-PC4200
it had 512MB in it whated to put another 1GB. Now when I put in the GB I go to controll panel>system and 980 MB show up not 1480 or so. when I have just the 512 MB in 480 MB show up. when I have just the 1GB in 980 MB show up. I have switch slots. So my question does anyone know why both won’t show up together. Both are DDR2-PC4200