Paginated Report previous report difference

I am trying to get the difference between months having the last 3 months as columns and the codes as rows. The issue I am having is that if a code has an amount in January and March, but in February is null/blank. How I need it to work is that in February it should give me the difference from the prior month (January) but the only difference it is giving me is the difference from the amount in January in March not the January in February.

Highlighted are the areas that have to be figured out