Walls Changing Length When Connecting with Angled Wall

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I'm planning to remodel my house and have the original house plans.  I'm trying to put the plans into Home Designer Pro 2015 so that I can then try various remodeling floor plans.  The house plans have an interior wall with a parallel exterior wall that define a garage.  Each wall has a different length called out on the plans, with corresponding ends of the walls being joined by an interior wall at the non-garage door end of the garage.  When I try to draw a new interior wall between corresponding ends of the parallel interior and exterior walls (the exterior wall being the  longer of the two walls in case it makes a difference), the wall lengths of the parallel interior and exterior walls change.  Sometimes, the distance between the parallel walls also changes.  I've tried locking the ends of the parallel walls in so many different combinations over a number of days that I now can't remember what I've tried and haven't tried.  I've tried other things as well, but haven't come across a fix that allows me to draw a new interior wall between the ends of the parallel walls that doesn't cause the lengths of one or both of the parallel walls to change.  The angle of the new interior wall relative to the exterior wall is around 50 degrees (per some trigonometry calculations based on the wall lengths called out on the house plans).  Has anyone had this problem and/or know how to solve it?  I'm thinking it must be caused by one or more settings that I don't have configured correctly, but am not sure if that's the cause or not.