2003 Lexus SC 430 electric antena mast

I'm trying to replace the antena mast as it will not go all the way down. I bought a new one from Lexus. I unscrewed the nut that hold the antena and tyry to pull out the ols mast but foundit very hard so gave up being afraid to cause any damage. Antena motor works good as antena does go up but not down all the way. Please tell me the way to fix this issue.