It could be one of two things, in my opinion:
1) Antenna Aim is off
2) Your Fresnel is "too good". If your Fresnel is more than 1.0, you will have a reduction in signal strength (depending on the Fresnel). I have seen many cases where antennas were too high for a particular location, resulting in too good of a Fresnel. You may need to lower your antennas, especially with such a short link and such tall towers. I suggest using Radio Mobile
to determine optimum antenna placement. It can also help you with azimuth readings for aiming antennas.
Here is an example - note the "RX Level" number, this is the predicted receive signal at the "Receiver" side.
"Too Much" Fresnel
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