more than likely your battery's have gotten sulphated to the point that they can not really hold a charge anymore. All of todays lead acid type batteries are very prone to do this when you dont keep them on a trickle charger..
If you need a quick fix on a tight budget here is a method to desulphate batteries using empsom salt. (it works very well)
he has a series of 6 video's showing the process from start to finish.
After you get them charged again, keep them on a trickle charger...... even better is to use a desulphating trickle charger that will keep your battery at its full potential for much much longer. I have used an older model like this one
http://batteryminders.com/details.php?prod=2012 ,and have lead acid batteries over 6 years old that i run for my trolling motor that still perform flawlessly. (they test out over 12.8 volts and I have never run em down even when using the trolling motor all day)