How do I stop one part of my window from frosting up??

Call your window supplier back. Sounds like there is either an installation problem, or the seal is broken. Chances are it can be repaired.

Lillie: have you checked the seal on the window. ...also maybe the window still may be under warranty -- and you may want to check with the manufacturer.

If they are storm windows the seal between the two glasses must have broken.

Weatherize your w indows when it gets colder. Make sure storm windows are installed properly and caulk any gaps in the seals. Keep your home sufficiently warm, especially at night, to keep frost from forming. It might be worth putting a space heater in a room that is particularly prone to collecting frost on windows to see if that solves the problem.

– If all else fails, you may choose to replace single-paned windows with double-paned windows. The barrier of dead air between the double panes helps to slow down the transfer of heat/cold.

https://www.youtube.com/watch?v=hew_seBHSMM

Try this video or call a window repair company. If your windows are only a few years old they should be under warranty.