I have the same problem so I am following this thread - but anyway, I'll take a gander but keep in mind I'm not a plumber or handywoman. I imagine you checked the obvious - the valve in basement and maybe behind your tub opened up all the way. Otherwise, it may be that the tub line is run off of a half inch line that runs to your sink. If that's the problem, I'm guessing you will have to run a separate line to your tub from the basement off a hopefully larger feed. A possible simpler fix is to buy an expensive shower head. At my last house, I put a Moen in that cost over $150 and I never regretted it. Good luck - I haven't solved my problem yet -cant' even get the door open behind my tub to chekc the valve.

After shutting off the water to this shower disassemble the shower body check for a lose washer or other debris that may have blocked the H/W passage way,

You may have to turn the water on when the shower is apart to flush out the line,