Try with more acid or change the acid (e.g. ascorbic acid).

 You can try heat the solution 600c for 10 of 15 minutes

Better to add 50 ml acetic acid for 900 ml distilled water( total makeup to 1000 ml with chitosan) and should be magnetic stirred at 70 C for 30 min

you can dissolve 1gr chitosan in10 ml acetic acid2% on magnetic stirrer without heating.