Can medical or all deductions get so high that the refund meter stops increasing? (I'm not using Standard Deduct; MFJ)

I just now subtracted $8000 randomly from $42,000 med expenses, and taxes refunded amount didn't drop.  So I guess there IS a ceiling, based on total income??

Ever since my refund dropped due to ACA subsidy I have to repay, it seems to have locked the refund at one small amount.

What's the deal???