Either the batteries are duff (people commonly try and use old faulty batteries for such purposes), or (more likely) the solar panels are too small to keep the batteries charged up - again, it's common for people to hugely over estimate the capability of solar panels, and the imaginary specifications provided by the sellers/manufcaturers doesn't help.
How big are the panels?, and what is their 'claimed' output?.
Basically you want LARGE panels, aligned correctly on the sun (you don't need to steer it, just point it at the sun at midday), and a proper charging system.
What kind of charger are you using as well?.