In the absence of test equipment there is no way to know for certain what went wrong, or what to do about it. The duplexer sounds like the most likely culprit, but the problem could be something else, like inadequate shielding in the receiver.
If you ask around, maybe someone in your area has the necessary test equipment and wouldn't mind helping.
An earlier Answer mentioned LNAs would be “of some use only if ... the noise power from the antenna is higher than the noise floor of your receiver” (quote). That statement appears in need of some clarification. If the receiver, on its own, has a high noise figure, or is preceded by a (long) lossy feedline (which contributes/adds to the system noise figure) ...
A LNA amplifies signal and noise. It is of some use only if:
the noise power from the antenna is higher than the noise floor of your receiver, and
the LNA has a lower noise floor.
Many VHF/UHF receivers already have sufficiently good noise performance that one or both of these things is not true, and so adding an LNA will not improve performance. Typically,...