yoharv;
Are you sure you have the batteries in series [check voltage]?
Do other things work on same setup?
Does the inverter complain, beep or whatever?
How big is the cable from batteries to the plug?
Are the connections solid and properly done?
Just the first few questions that pop up.
Should give you a place to start tracking it down.
Remote troubleshooting is a bit hard without a LOT of info you have not provided.
Cheers.
TomW