You are using the global transposition for horn players instead of the regular transposition.

First you should turn off global transposition for horn players:
• iOS: On the top of your screen in song view, tap on the third button from the right, Aa, (Chord Chart Settings) , then Transposing instrument and select C.
• Android: in the app under More, tap Transposing Instrument and choose C.
• OS X: in the app Preferences, choose C in the Transposing Instrument drop down menu.

Then you can transpose normally:
• iOS: Tap on the chart, then the key in the player controls at the bottom and select a new key.
• Android: Tap the Key on the left and select a new key.
• OS X: Click on the key in the Player controls (right top) (or bottom right if Player controls are minimized) and choose a new key.