I agree, the difference in splitter specifications don't explain 'doesn't work' vs. 'does work'. In addition to the possibility of a bad splitter, bad connectors come to mind. The unused splitter port needs to be terminated with a 75Ω resistor cap, either at the splitter or at the unused jack at the far end of the coax.
Mounting indoors, in the loft or attic, will limit reception of any antenna.
The Winegard antennas are appropriate for the conditions represented in your TVFR. To take full advantage of a single antenna's capabilities, a rotator and preamp plus outdoor mounting would be needed.
My personal inclination is to want every possible signal. To that end I'd consider any of the large consumer grade antennas, including: the Winegard HD7698P or the combination of an Antennas Direct 91XG and an Antennacraft Y10713. I'd also add a preamp. The Antennas Direct CPA-19 is a top pick.
If you're a skilled antenna builder, the GH10 is worth looking into.
It doesn't sound as if you're inclined to build an antenna farm. But if you are willing to consider a multiple antenna installation, there are a variety of options worth considering which would eliminate the need for a rotator.