There are numerous possibilities here. Again, which one did you get? And which RS scanner are you comparing against?
Based on the limited answers, here are some thoughts..
The STARS system is, I think, simulcast, and most all of the consumer scanners can't handle these systems very well. (The Uniden SDS100 is too new to say whether for sure whether it solves this issue). Basically, scanners can't handle digital signals that carry exactly the same information from several directions well (yes, that's highly oversimplified, for the techies among us). This is the curse of simulcasted systems - commercial radios are built to handle it, consumer scanners not so much. This is referred to as simulcast distortion.
There are some ways to improve it somewhat - using a small beam antenna pointed at your nearest tower is one way. A 2 meter ham beam would work acceptably in this. However, these can be a bit big to put in an apartment with no attic or space to put them in. Even a smaller antenna like a small 2 meter ht antenna (not a 5/8 wave) might actually be a better choice. Here is an example...
https://www.universal-radio.com/catalog/hamantht/3938.html
Put the attenuator on the system. If it improves reception - even by a bit - then there is a chance that simulcast distortion is the problem. Without knowing which software you were using, I can only point you to our wiki, where we have user level guides for ARC500 and Win500 linked at the bottom of...
Programming Your GRE Scanner - The RadioReference Wiki
Are you sure of the site(s) you are programming? Perhaps there's a better site for you to use. As I'm not in Virginia, I will point you to our Virginia forum...
https://forums.radioreference.com/virginia-radio-discussion-forum/
Post a question there with STARS in the title, and there will be folks to help.
Finally it's been long known that the GRE scanners (which Whistler bought out and rebranded with their model numbers) suffer from overloading from local FM broadcast stations. This can cause the scanners to become less sensitive in this band than they would otherwise be (this is loosely referred to as desensing). If you use an external antenna and find your reception has actually decreased, a FM broadcast filter may be the answer. Places like PAR Electronics (the owner of the company is / was a RR member) carry them. They can be a bit expensive but you get what you pay for.
Mike