Might help if you let us know how your trying to connect, NMEA 0183 or NMEA 2000 or other.
Common mistakes could be wrong wire connceted somewhere, or baud rate not set correctly, the VHF may want to communicate at a lower rate than you have set or is default on the port of the plotter.
If it were me I would look closer at both manuals for the two bits of kit, find the wiring instructions and simply do a quick drawing on paper, which wires to which wires, which port used and what baud rate, then triple check wiring connceted and baud rate set and take it from there. Doing a drawing really does help as there can be a lot of wires.
And remember output goes to input and input goes to outport.
I recently connceted a VHF set, a Plotter, A AIS and a heading compass after doing a full wiring drawing and it all worked first time which was amazing as different manufacturers of kit and hence different wire colours.