I forgot to include additional medical expense but when I add it in now it makes no difference to refund amount. Why?

[TonyC11 · Accepted solution]

There are several possibilities. More than likely it is because your credits before adding the missing expense were already sufficient to reduce your tax on taxable income to zero.
Since medical expense is non-refundable credit, you  get no extra tax refund.
Consider holding the extra expense for a claim next year.