Trouble with OTA antenna using diplexers

[drf]

And I have the 18db amplifier that comes with the antenna plugged in after the last diplexer, right before it goes to the receiver.

Check the manual that came with your antenna. It specifically states in the 'Important Information' section:
It is essential for proper operation of this antenna system that
the amplifier be connected between the antenna and any devices
such as splitters, matching transformers, networks, etc.

So you are putting the amplifier in the wrong place. It needs to be on the antenna side of the diplexors.

 

[reservoirreb]

Without both, I don't get a great signal..

for every channel, but I do get some channels still very well.

The amplifier helps out a lot, but it doesn't have to be there from some channels.