Undocking problem Need Help

[SuperCookie78]

At the moment in my kerbal career save i am trying to expand my space station around minmus. I have tbe first 1/4 of my circler section parked 140m away from my station. When i when to go get my  module tug it didn't want to undock. I clicked undock in the menu for the docking port and the menu only shows enable/disable crossfeed and something else that i can't remember as i am in bed right now. The tug is still attached to the station and i can transfer crew in and out of the tug as if it were still attached to the station. I know that i can dock two spacecraft because i sent a probe with mechjeb on it to the same station. Can any of you help me? Its my first time on the forums so hello KSP forum:)