Hmmmm.
My group sort of has this issue as well. Two of us have characters with history and personality, the other three just seem to play for combat. However, for our group this makes for a good mix. Personally, I think gaming should be an equal balance between role-playing and roll-playing.
When I created my character, I also made up a list of friends and family, gave her an interesting history (that the GM has free reign to play around with...so it could come back to haunt me....

) as well as just general character traits I wanted her to have. Her horse and her familiar have their own names and personalities as well...(and my GM has *hinted* that because my character treats her horse as more than just a means of transportation, this may pay off later on down the line... That's not why i did it. I did it because I truly enjoying creating characters, however, incentives are a good thing for the unmotivated... )
Perhaps asking your players to each write a brief history of their character and examine what their goals are. Or run a session that is light on combat and heavy on personality and see what happens.
Oh man...I sound like a guidance counselor...
But seriously, I found that I became much more into playing Djeta after I came up with her past. Made me care about her more. Ya know?
~Sheri