QUICK ANSWER
Press the bumper across its full width and make sure it springs back evenly.
iRobot’s bumper guidance says persistent bumper errors can come from trapped debris or a bumper that has become dislodged on one side.

INTERACTIVE DIAGNOSIS
Find the bumper fault
STEP 1 OF 2
Does the bumper return freely and sit evenly on both sides?
Press the center and both corners several times.
Avoid forcing the bumper shell
If a side is visibly dislodged, follow the official procedure or use service rather than bending the plastic into place.