Lots of possible causes:
Weather can affect the signal if you're a long way from the transmitter.
There may be several transmitters providing the signals you're watching, some "head on" to the aerial, some off the side.
Some transmitters run more power than others.
A splitter without an amplifier shares the signal between all the sets. When you add a new TV, you reduce the signal going to all the others.
If the aerial's in good condition and pointing the right way, then a masthead preamplifier is the way to go. I installed a new aerial here when we had the house repairs done. I included a preamp.
We get perfect TV on all 99 channels, not there's much on any of them that I watch.