All of the photos you posted, except the last one, have a strong backlight, and are shot with the Pattern metering mode. This "Measures the light at points across the entire image to achieve a balanced exposure setting", but since the light from the windows in the background consititute a major source of light, the rest of the image (including the subject) is quite dark. The Panasonic FS5 has a 'Backlight Compensation', but only in the 'Intelligent AUTO'Shooting mode. If you're not using the 'Intelligent AUTO'Shooting mode, perhaps you should try, and if you are, perhaps it isn't compensating enough and you should try to avoid strong backlight situations.
I notice that you're using a high ISO setting (400-800), and that could be making the situation worse. But I also notice that you're using large apertures (relatively speaking) and slow shutter speeds. Maybe you should use the flash. This will get you faster shutter speeds and better lighting for the foreground subjects.