IRA - RMD

I have a regular IRA funded over the years completely by after tax contributions.  Last year when I tried to enter cost the of the RMD which was not known by the bank it had no effect on the tax.  After several tries I filed it with zero cost.  How can this be?  This year I would like to enter the cost and see it reduce the tax on the RMD.  Could you please comment?  Many thanks, Len