SJSO
Try taking your scanner remote. Get closer to the site, even with no antenna attached, and see if you get clear reception. That will help ID whether you have a reception problem or digital decoding problem.
Do you know if you are near any cell towers or other (high powered) UHF transmitters? They could be overwhelming your receiver. Try receiving with the attenuator on. If you are using an 800MHz antenna, go back to the stock antenna. Make some changes, note the differences, and that will help ID what your underlying problems are.
When SJ was analog, I'd get the main channel all the time, but not the secondary channels (I'd get them at night only). Now that they're digital, I am amazed at how clear everything is 24/7. Although the digital sounds like it is going through a warbler or flanger sometimes. But that happens on most digital signals from time to time.