IF I understand it correctly, you THINK that the Hi-VHF Loop+ScreenReflector has poor F/B Ratio performance because you pick up a mostly OVER-WATER (Ch8 at 256-deg???) station [which can benefit GREATLY from DUCTING, esp THIS time of year] better than those completely over LAND???? Is there any other indication of poor F/B Ratio Performance??? What happens when you rotate the Antenna 180-deg in the opposite direction??? I tend to agree with your guess that it's the "Weather"....
It's probably not an issue, but is it possible that the 1/4-in Screen Material is NOT made of conductive metal??? IF in doubt, perhaps you can check it with an VOM Meter.
I don't think you have enough Antenna GAIN to pick up Hi-VHF Ch7 (NBC, NM=8.9 dB) and Ch8 (CBS, NM=3.0)....but before upgrading the Hi-VHF Antenna, there are some other issues that need to be addressed.
If you are still using the AntennaCraft 10G212 Preamp with extremely HIGH 30dB Gain...THAT can be a BIG problem for receiving weak Hi-VHF Stations. Although FMFool is also off-line at the moment, I can see that there are numerous FM stations within a dozen miles of your zipcode....the second Harmonics of which fall on-top of various Hi-VHF Band channels....very strong Ch13 and strong Ch11 can also generate 3rd Order Intermod Distortion products on top of other Hi-VHF Channels.
1) First, you should insert a FM FULL-BAND FILTER between the Hi-VHF Antenna and the Preamp, either from Antennas Direct or MCM:
https://www.antennasdirect.com/store/FM_band_rejection_filter.html
FM Trap | FM-88 (FM88) | Distributed By MCM
2) You might also need to replace the A-C Preamp (which has probably ALSO been degraded by the weather) with a Very High Overload Preamp, such as the Antennas Direct Juice:
https://www.antennasdirect.com/juice.html
Or alternatively, the inexpensive RCA TVPRAMP1 or Channel Master CM-7778.
3) Hi-VHF Antenna Upgrade: It was unclear whether you were using a Single or Vertically Stacked Loops with Winegard SD 3700 RF Combiner...which is it???? If pointed to receive Ch7 & 8, Ch13 (and maybe Ch11) will probably come in anyway and Ch9 would be a duplicate to Ch8, so I'm guessing that you'll only need a High Gain Hi-VHF Antenna pointed in ONE Direction.
Note that I posted Vertically Stacked Hi-VHF Loop+ScreenReflector designs that eliminate the Loss in the RF Combiner, providing a bit more than 10 dBi Raw Gain (as much as the obsolete Y10-7-13 or YA1713):
V-Stack Hi-VHF Circular Loops
Also note that I posted the simple to build Hi-VHF Hourglass-Loop+ReflectorRods designs that provide nearly 10 dBi Raw Gain, which would be about 13 dBi Raw Gain in a Vertical or Horizontal Stack....interconnected by a Vertical (HVH) or Horizontal (HHH) Harness that I would have to run through nikiml's Optimizer if you chose to go that route. And they are relatively insensitive to Wire Size, unlike Hi-VHF Loops:
http://imageevent.com/holl_ands/loops/hivhfhourglassloop