are you sure that you are using the right amount while considering the slightly acidic media for chitosan dissolution? which salt of chitosan are you using? the grafting process is highly dependent on the resultant hydrogel one way for calculating the grafting value which might not be as accurate as the other methods is to weigh the polymers and the crosslinker if it is applied and then reduce the initial weight from the final weight, so if one polymer is not fully dissolved there is an overt relation between the grafting percentage.