Installing new dishwasher and want to use existing copper tubing(10 yrs old), cut off end tried new compression nut and ferrule but it won't fit, I'm sure it's 3/8 but it won't fit, did the tubing expand? What to do to get it to fit? Thanks

Is it really close to fitting, or is it way off. If its close, then you will need to just cut back further to get to a straight piece of tubing. If its way off, I would guess that the supply copper is 1/2" o.d.(outside diameter) copper. I personally would suggest getting rid of the 3/8 or 1/2 od copper line and using a stainless steel braided flexible dishwasher supply.