There are a few possibility why amoxicillin has not help.  The antibiotics may not work well against the bacterial. The duration may not be long enough.  There may be other causes to your sinusitis, such as uncontrolled allergic rhinitis, foreign body, polyps or even a dental cause.  Hence the treatment of persistent or prolong sinusitis is to treat the underlying cause.  Antibiotics is part of the armamentarium against sinusitis.  You may consider sinus rinse, improve the control of allergic rhinitis (if any) and seek additional help from your family doctor or an ENT surgeon.