Non conductive mast ? That's what you need and you need a ground plane which is typically 4 x 102 inch radials at 45 degrees on the "earth" side of the coax otherwise it will not be a match to 50 Ohms.
If you have radials and a non conducting mast it is something close by to the antenna knocking it out. Maybe a gutter, water tank ? In your or neighbours house.
If you are unlucky some concrete has mild steel rods in it to strengthen, even a close, sloping wet roof wall can do it.
A photo or 2 would be very helpful as we cannot know how you have configured this tank whip presumably on a 3/8 inch mount of some kind. Some 1/4 waves can be mounted on a specific size pole so can probably be put on metal pole but I would tend not to if I had a choice.
There is a slight chance if you do not have any radials and put a 102 inch whip on the garden grass it may have an ok SWR. An ok SWR is a good start but it does not mean it is a good antenna as the ground itself will be potentially acting as a very lossy ground plane. However, I suspect this is not the case and you have some other installation issue.
Photo's will help.
Today at 7:59 am by 
Subject: High SWR issues 
