Switching leathers is a great idea to keep them the same length. I do this everytime I clean my tack.
You can take your leathers off your saddle, place them on the floor and see if the holes match up along with length. It might just be a simple tack issue.
I start off my lessons with my feet out of the irons for a few rounds around the arena. It allows your legs to stretch before you go into work. This helped with my issue of having one foot from coming out of the irons. Mind your balance, you may lean to one side which can cause your foot to come out. We all naturally favor one side.
All of the suggestions above in the previous posts have great suggestions.
Let us know how things work out.