If you are not getting the range you expect from your handhelds, or your coverage varies with weather you might be suffering from moisture in your coax connector at the base of the antenna. I has been my experience one wrap of plastic tape is not enough to prevent moisture from entering the connector. Your antenna has an N-type connector and one drop of water will cause reduced performance. I suggest one wrap of quality plastic tape such as Scotch 33, then two wraps of 30-mil rubber tape usually used for wrapping high-voltage connections, like Scotch 130C or 2242.
Regarding your transmission line, for any full-duplex or repeater use, avoid coax like LMR400 as it will actually create receiver noise due to the shield design. Professional installations only use Heliax cable that has a solid copper jacket with plastic cover. Usually, 1/2 inch cable is acceptable under 120 feet length at VHF.
Also, when using duplexers it is strongly suggested only using jumpers made with double-shielded silver-plated cables such as RG-214 for large cable or RG-142 for small cable. AND it is essential you also use quality silver-plated connectors on all jumpers between the duplexer and the radio. "Leaky connectors never look bad"
Last, do not attempt to "tune" your duplexer without the proper test equipment.
To test your system for "desense," communicate from the repeater with a local microphone to a handheld at the edge of coverage, and listen to the handheld count to 20 on the local repeater speaker, while switching the "repeate disable" switch on and off [which simply turns the transmitter on and off]. If all is well, there should be not noticeable difference receiving a distant signal, regardless of the transmitter being on or off. If there is a difference, then some of your repeater's transmitter noise is finding it's way into the receiver and causing "desense," which reduces your coverage. Professionals usually use a signal generator with a variable output connected to another outdoor antenna at the repeater site to make this test. Modulating it with a tone makes it easier for quick tests.