There are 5 tie down points on the top. It sounds like the rear 2 are not "disengaged". Be sure all 4 tie down spots ore accepting the "bayonettes" then tighten them down before securing the rear ratchet hook.

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si