when I make scalloped potatoes they never cook up right ,sometimes they cook all the through in maybe 1hr, then next time in 2hrs and they arent cooked yet. I think it is the potatoes that I must be useing at the time.
For the potatoes to cook, they need to be in the middle of the oven at the same consistent temperture that you had them when you first cooked them the right way. The size should not differ. If they are too big, you should cut them.
I didn't see your earlire question but I'll try to help.
When you make your scalloped potatoes, do you always cut them the same size? the bigger or thicker the potato the longer your cook time. You always want them the same size, the same amount and the same size or shape dish. If you're making twice as much, its obviously going to take longer.
Different potatoes really do take different longer or shorter to cook, too, though it shouldn't affect your cook time that much. I prefer Russet potatoes over pretty much any other because I like the way they cook up and the way they taste vs baby reds or Yukon Gold. Still, the difference in cook top should be minimal.
I don't know if this is an option for you but my mother swears by this and I love her Scalloped potatoes more than anyone elses. She mixes up the whole casserole dish (she uses a round Tupperware dish but I suppose this would work with whatever dish out used) and she microwaves the potatoes for about 5 minutes on about 70% power and then she sticks it in the oven for the rest of the time. All this does is get the "juices" hot and get the raw potatoes hot but when its all done baking...MMMM! I tried making this once and didn't microwave...It didn't take anything close!