The VHF owner's manual shd tell you what NMEA version and which sentence(s) the radio
is looking for. Usually it is the CGA and/or RMC. Then check the your chartplotter
and see if it is generating the required sentence.
If your chartplotter
does not generate the required sentence then you may need a different NMEA source for the VHF. An NMEA multiplexor can solve the problem. Raymarine makes one as do several other companies, e.g., Brookhouse.
The problem could also be the NMEA version generated by your charplotter. I don't know what if any changes may have been implemented in the various versions, but many of the listening devices, such as DSC
radios, are very picky about validating the sentences. If any field is missing or doesn't conform to the latest standard the sentence is rejected.