What would cause the loss of relative activity in an enzyme over time? And how would this factor in when you use elution and wash buffers and dialyze your sample? Could this be reduced by being more careful with how much buffer you add?

That is a very broad question, and it depends very much on the enzyme in question. Activity may be lost over time because the enzyme is oxidizing or degrading in some way. Also, if it adsorbs onto surfaces, you can lose some activity that way. The amount of buffer would not be my first place to look, but any calculations of activity would have to take into account differences in volumes from one experiment to the next.