you should  look at other methods  for synthesis GO . in the literature  there is an improved method for the preparation of graphene oxide (GO) is described. Currently, 

Hummers' method (KMnO4, NaNO3, H2SO4) is the most common method used for 

preparing graphene oxide. We have found that excluding the NaNO.  This method published  by

DC Marcano, DV Kosynkin, JM Berlin, A Sinitskii… - ACS …, 2010 - ACS Publications

If you are only seeing one peak at 1635 cm-1 in  FTIR , then most likely your graphite hasn't oxidised. Compare the FTIR of your graphite starting material with the GO obtained. The peak at 1635 cm-1 is most likely due to O-H bending from water as well as some C=C stretching contribution. 

Most probably you are doing something wrong during your oxidation steps in the very beginning - the washing step, if done incorrectly, should just leave your samples with a high degree of contaminant residues. The Marcano-Tour method works just fine, and it has even higher yields than the improved Hummers; it has gradually become the preferred standard for GO preparation, and I wouldn't advise you to switch back to the Hummers - just try to perfect your Marcano-Tour. 

My guess is that most probably you did not control the temperature properly during the oxidation step: if it raises too much during your exothermal reactions, any produced GO gets reduced back to graphite. This might explain your partial oxidation.

FTIR can only give you part of the answer. I would advise you looking at the Raman spectra as well.

From what has been described, I highly doubt that the problem lies in the oxidation step. Maybe the agitation was not enough.