It sounds like Medicaid has figured the income allowance for your mother correctly and that she is getting the maximum allowed at this stage. However, your mother can appeal the income allowance and show that she needs more funds. Each state has different regulations for this, but you would probably need to show that she needs to be in assisted living for medical reasons. You will need to consult with a Tennessee elder law attorney to determine whether such an appeal would work for your mother and you will probably need to retain an attorney to represent her on the appeal.
For more on Medicaid's protections for the spouses of nursing home residents, click here and here.