If there's a problem, it could be the antenna, the cables
/ connectors, and/or the radio. An SWR meter will catch many of these problems. Find someone who knows how to do these tests. We can talk you through it here, but it's *much* better to get some local help.
I would also do some listening tests. Find a weak (noisy) signal and compare your two radios. The masthead antenna should definitely outperform the handheld.
Also, see if the SEATOW automated VHF testing service
is available in your area: https://www.seatow.com/tools-and-edu...ed-radio-check
With this you can hear an on-the-air playback of your signal, which tests both your transmit and receive.