A Defective Transfer is the result of a weak application of the transfer roller to the plate. Part of the design may be very faint or missing competely. (Image provided by RVS)
A Re-entry (AKA "Double Transfer") may be the result of trying to correct a Defective Transfer with a second application of the transfer roller to the plate. Doubled lines can occur when the second application does not perfectly align with the first.
A Retouch may also be the result of trying to correct a Defective Transfer. In this instance, an engraving tool is applied to the plate in an attempt to make the weak lines deeper into the plate and thus darker.
A Plate Flaw is usually the result of damage to the printing plate that caused an indentation. This area would fill with ink during the printing process and leave unwanted mark(s) on the stamp.