Dehydrating Bananas?

I just purchased a dehydrator and have been successful with the few items I have dehydrated except bananas. This is the 2nd time and each time, they never got to the "chip" consistency. First time, I did nothing special to them except slice and dehydrate, 2nd time, I used a preservative and still after 13 hrs, they have never gotten to that "chip" consistency. What am I doing wrong? I have dehydrated at 135 and 145 degrees and still did not get there. Anyone have suggestions? I would very much appreciate as I love banana chips.

By Elaine M P.