IBM 6400 model i10 print problem

It's likely a "hammer" is defective. This can be caused by
a build up of ink under the ribbon shield and/or around the hammer "fret" itself or the fret is physically broken. Remember
that this is a "shuttle" printer and each fret covers anywhere from
one to three characters depending on model. Some cover fractions of characters as well. The problem will appear as either
data missing on the left or right margin or a blank column of missing data going down the page.

A technician is required to solve all of these problems as the
shuttle assembly must be removed.