Lighting – I need help opening this light cover

Pull the spring-loaded prong away from the dome, and tip the dome below it - then pull the dome away from the other two prongs. Don't push the spring-loaded prong with the dome.

• Simple geometry says that the required distance of movement to get one prong clear is considerably less than what's need to clear the ones at 120 degrees from it.
• Trigonometry quantifies that to about half, if I'm constructing the problem correctly in my head.

Sounds like you have a connection problem. Ever try pushing on the light without installing cover to see if it will flicker or anything like that? This would indicate a loose wire connection.