It's very hard to say. My best guess would be that there is something inside the carriage that is causing it. To find out would require opening the case of the carriage, which is not recommended for the average user. Here's something to try:
- Remove the carriage from the machine.
- Open the arm until it locks in the 90 degree position.
- Hold the carriage to the open side of the arm is facing down and shake it. If there is anything loose inside it might fall out.
- Use a strong light and look in through the opening on the arm to see if there is anything obvious, like lint, a bit of fabric, pin, etc. You may be able to use tweezers to carefully extract it.
Otherwise you may want to have your dealer open it up and see what's happening inside.