Depending on your software version a VSWR alarm/reduced coverage alert will get triggered when the VHF antenna is not performing optimally. A good VSWR is typically 3:1 or less. VSWR can be caused by a faulty or loose connection at the connectors, cable, or the VHF antenna itself.
It can also be caused when a VHF antenna is mounted parallel to a metal surface, or when in close proximity to an obstruction or another transmitting antenna.
It may also be possible that your VHF antenna is unsuitable for AIS. AIS operates around at frequencies around 162MHz, so if your VHF antenna isn’t tuned accordingly it will not receive or transmit as far as it could do.