After some light cleaning I was able to get Prosper FD, which is the FD closest to me. They are dispatched on Frisco's P25 single site. The audio was very faint even with five signal bars.
It doesn't look your window has any tinting. The tinting films can reflect a signal. I can't tell how far the center pin of the magnetic mounted antenna connector extends outward from the photo. It should be about even with the insulated portion of the connector. It is also possible that the RF amp of the scanner may have been damaged by exposure to a strong signal from a nearby transmitter. Did you recently key up a transmitter close to the scanner or were standing close to any public safety radios when they keyed up? Something you could try is a Remtronix REM-800B antenna. That is high gain on the 700 and 800 MHz frequencies used by the trunked systems of Denton and Collin Counties and works much better that the stock antenna. It will work OK on UHF, but VHF frequencies will suffer.
Are the pins on the magnetic mount and the stock antennas about the same?The pin is recessed in. Is there any way to fix that?
Are the pins on the magnetic mount and the stock antennas about the same?
On the magnetic mount antenna, try holding the BNC connector in one hand and pushing the cable into it with the other. See if it extends the center pin.
The more systems you monitor, the less you hear, especially multiple trunked systems. While scanning one system, you are missing everything on the other systems.
It can also be caused by the foliage on trees. In winter, trees are bare of leaves and offer minimum attenuation of the signal. In summer, the leaves give additional attenuation of the signal, especially wet leaves. I have had a lot of experience with TV live trucks. There have been a lot of locations where we could get usable signals during the winter but were hopeless in summer.
There may be only one tree in your backyard, but what is between your house and the transmitter sites? Most likely, any increase in your antenna height will result in better reception. Using LMR400 cable will minimize transmission line loss which can be very significant at 800 MHz when using RG58 coax.
If a CB 27 MHz antenna is working better, that is a good indication something is wrong with your magetic mount antenna. What is the frequency coverage of the magnetic mount antenna? Is the CB antenna a base or mobile antenna or is an antenna off of a handheld?
I believe the REM-800B antenna may be the answer in your situation. There are 700/800 MHz systems I could not receive with the stock antenna which work reliably with the Remtronix antenna. Your scanner magnetic mount antenna is either defective or works very poorly on the 700/800 MHz frequencies. The REM-800B is only 7" long so it will be very convenient to use with your scanner. You could even have a BNC connector mounted to your window and use the Remtronix antenna.