Could be several things but if you're shooting JPG you may want to adjust the Sharpness setting in the Picture Control menu.
To make this adjustment...
Press the Menu button.
Highlight the Shooting Menu (camera icon)
Drop down to "Set Picture Control" then click right one time.
(This menu contains all the Picture Controls (Stanadard, Vivid, Landscape, etc. and each one has it's own set of Quick Adjust menus)
Highlight the Picture Control you're using, probably "Standard" and click right one time to enter the Quick Adjust menu.
Highlight the "Sharpening" setting and increase it from the oddly low default to +6 or +7.
Press "OK" to exit the menu.
This one adjustment will have a significant impact on the overall sharpness of your JPG photos.
....