Have you checked the binding affinity of protein A/G to your type of antibody? Some antibodies will not even bind to those, like chicken antibodies. Take a look at this list from Pierce: http://static.fishersci.com/cmsassets/downloads/segment/Scientific/pdf/Antibodies/binding.pdf

If that is not the case, then you may try to change the time and temperature of binding. I use 30 min at room temperature with rotation.

All the best.