Another thing to check on the BQ-270 is for a sheet metal support on the lower front clamp of the carriage clamp. It is made somewhat like a piece of angle iron, but it is made of thin steel. It's maybe 3/8" in height and the full length of the front of the carriage clamp.
It is held on by four screws and is visible (and removable) when you put the carriage clamp over the nipper/cover clamp station, you have to get on your knees and look under the carriage clamp.
Essentially, this piece of sheet metal is an expendable item that is meant to bend or get mis-aligned if an improperly loaded book block gets through to the nipper. Once bent or mis-aligned, book blocks don't clamp well when the cover is clamped on as this sheet metal piece is a lower extension of the carriage clamp.
The part is called a sub-plate and is available from your local Standard Horizon service provider.