if you are receiving some data at 2m, that minimises problems with the unit.
Your aerial height is sufficient for 20nm range on larger merchaant ships
That means that the most likely culprit is the aerial and its connection. I would check your connections and make sure that you have not cross connected the earth wire into the aerial
Please dont call this a radar, it is an AIS
receiver. NASA should be shot for using this name.
I had no problems with connection using a 1.1m stainless whip.
However, I set my system up so that I could swap over the VHF
aerial to the NASA aerial, and the AIS
to the masthead aerial. - redundancy, and another way to check it.