My first guess is you would get better results with UHF and dedicated UHF antennas for transmitting and receiving, but I suppose you have some VHF only receivers. In that case, I would expect the upper VHF channels to work better. Are you seeing differences due to channel number? Have you tried adjusting the length of the dipoles?
There have been discussions here on what kind of antenna to use to transmit - give it a search.
Combiner manual says 17 dB insertion loss, so you need a 17 dB amplifier following it to get back to the modulator output level. It also needs to be able to handle the desired signal level without crossmodulation, so you need a 20 dB gain distribution amp designed for the high signal level input; a 20 dB antenna distribution preamp may have problems.
Manual:
https://cdn-docs.av-iq.com/other/PC1601_Datasheet.pdf