Water Flow Troubles

Greetings Gents, thanks in advance for an insight.

I have 2 bathtub/showers upstairs that are both exhibiting the same symptoms. Both have a hot knob and a cold knob and a knob in the middle (I believe it's called a deflector). When the deflector is turned all the way to one side, I may get 80% flow out of the shower and 20% out of the bathtub, when I turn it all the way the other way, I may get 20 & 20 out of the shower and bathtub. A couple of notes:

-My house is relatively new...built in 2000 and well built.
-There are a few houses in my neighborhood that are currently being built. I get some dirt in the spickets every now and then that I need to clean out
-Thinking it was clogged, I pulled off the shower heads and bathtub nozzles and cleaned them out. From one of the heads I pulled out what looked to be scraps of a black rubber seal/gasket. That improved the flow but did not solve the 'deflector' issue.
-When I pulled off the shower and bathtub heads, all that was exposed was the plumbing coming out of he wall...point being the only thing I could think of to try next was to rip out the wall and dig further.
-If I remember correctly, when I had the heads off and turned the water on with the deflector turned all the way to one side, the flow was significantly restricted...I'm thinking that something has to be blocking the water flow or the deflector is not opening the valve all the way.

Thoughts?

Thanks again

I'm very surprised with the valves relatively new they are the type you indicate...they would not meet code today as they don't appear to have any anti-scald features. But, the central valve is a divertor, and it sounds like it may need servicing or replacement. If you know or can find out the brand and model, you should be able to get parts since they aren't all that old. If they're an off brand, you may not find parts.

But, you might do yourself a favor and consider replacing it with one meeting todays codes...they are safer. This would prevent major temperature swings if someone say flushes a toilet while you are taking a shower.

diverter

IT is diverter, not deflector, and if you are even considering tearing up the wall to fix it, you are well beyond your level of incompetence and need a plumber. The black stuff may have been the diverter's seal washer and it is repairable without tearing your house down.