You need to redo those connections. There is a flat plate in the connection that should be above the wire and in contact with the screw. The wire should be between the flat plate and bottom of the connector with the screw forcing down the plate on the wire. Yes, stranded wire can be used in this breaker.
My "plate" moves with the screw so you'll have to check yours out.
(Adding as an edit) This is the plate being mentioned here. It should be between the wire and the screw:
Also make sure you followed the wire stripping gauge on the breaker and inserted the wires far enough into the breaker. Some breakers have a "frame" in the back of the connection to hold the wires in place so the screw doesn't spread them apart when torqued properly.