Is there a medical expense deduction limit, if income is below the threshold limiting medical expense deductions. a point when another $ of expense doesn't lower tax

I enter in turbo tax $15,000 dental expense, tax owed drops $1,000.  Enter another $650 in lab expense category, there is no change in the tax amount TT computes as owed.  Would expect some decrease in tax owed unless there is 1) a medical deduction limit, or 2)TT is broke.